Ursula Dronke (1997, Poetic Edda II, p. 144) writes:

"I have chosen to interpret Míms here as 'Heimdallr's' and his sons as 'mankind'. …If we take Míms synir as giants, then leika implies that they are rejoicing as mankind's, and the gods', fate approaches.

Codex Regius:

In the  immediately preceding strophe, Völuspá describes how it looks on the surface of Midgard when the end of the world is at hand. Brothers and near kinsmen slay each other. The sacred bonds of kinship are broken. It is the storm-age and the wolf-age. Men no longer spare or pity one another. Knives and axes rage. Hati with his companions have invaded the world, which it was the duty of the gods to protect. The storms are attended by eclipses of the sun (str. 41).
 
Then suddenly the Gjallar-horn sounds, announcing that the destruction of the world is now at hand, and just as the first notes of this trumpet penetrate the world, Mimir's sons spring up. "The old tree," the world-tree, groans and trembles. When Mimir's sons "spring up" Odin is engaged in conversation with the head of their father, his faithful adviser, in regard to the impending conflict, which is the last one in which the gods are to take a hand.
 
I shall here give reasons for the assumption that the blast from the Gjallar-horn wakes Mimir's sons from a sleep that has lasted through centuries, and that the Christian legend concerning the seven sleepers has its chief, if not its only, root in a Germanic myth which in the second half of the fifth or in the first half of the sixth century was changed into a legend. At that time large portions of the Germanic people had already been converted to Christianity: the Goths, Vandals, Gepidians, Rugians, Burgundians, and Swabians were Christians. Considerable parts of the Roman empire were settled by the Teutons or governed by their swords. The Franks were on the point of entering the Christian Church, and behind them the Alamannians and Longobardians. Their myths and sagas were reconstructed so far as they could be adapted to the new forms and ideas, and if they, more or less transformed, assumed the garb of a Christian legend, then this guise enabled them to travel to the utmost limits of Christendom; and if they also contained, as in the case here in question, ideas that were not entirely foreign to the Greek-Roman world, then they might the more easily acquire the right of Roman nativity.
 
In its oldest form the legend of "the seven sleepers" has the following outlines (Miraculorum Liber, VII., I. 92)[2]:
 

Seven brothers[3] have their place of rest near the city of Ephesus, and the story of them is as follows: In the time of the Emperor Decius, while the persecution of the Christians took place, seven men were captured and brought before the ruler. Their names were Maximianus, Malchus, Martinianus, Constantius, Dionysius, Joannes, and Serapion. All sorts of persuasion was attempted, but they would not yield. The emperor, who was pleased with their courteous manners, gave them time for reflection, so that they should not at once fall under the sentence of death. But they concealed themselves in a cave and remained there many days. Still, one of them went out to get provisions and attend to other necessary matters. But when the emperor returned to the same city, these men prayed to God, asking Him in His mercy to save them out of this danger, and when, lying on the ground, they had finished their prayers, they fell asleep. When the emperor learned that they were in the above-mentioned cave, he, under divine influence, commanded that the entrance of the cave should be closed with large stones, "for," said he, "as they are unwilling to offer sacrifices to our gods, they must perish there." While this transpired a Christian man had engraved the names of the seven men on a leaden tablet, and also their testimony in regard to their belief, and he had secretly laid the tablet in the entrance of the cave before the latter was closed. After many years, the congregations having secured peace and the Christian Theodosius having gained the imperial dignity, the false doctrine of the Sadducees, who denied resurrection, was spread among the people. At this time it happens that a citizen of Ephesus is about to make an enclosure for his sheep on the mountain in question, and for this purpose he loosens the stones at the entrance of the cave, so that the cave was opened, but without his becoming aware of what was concealed within. But the Lord sent a breath of life into the seven men and they arose. Thinking they had slept only one night, they sent one of their number, a youth, to buy food. When he came to the city gate he was astonished, for he saw the glorious sign of the Cross, and he heard people aver by the name of Christ. But when he produced his money, which was from the time of Decius, he was seized by the vendor, who insisted that he must have found secreted treasures from former times, and who, as the youth made a stout denial, brought him before the bishop and the judge. Pressed by them, he was forced to reveal his secret, and he conducted them to the cave where the men were. At the entrance the bishop then finds the leaden tablet, on which all that concerned their case was noted down, and when he had talked with the men a messenger was despatched to the Emperor Theodosius. He came and kneeled on the ground and worshipped them, and they said to the ruler: "Most august Augustus! There has sprung up a false doctrine which tries to turn the Christian people from the promises of God, claiming that there is no resurrection of the dead. In order that you may know that we are all to appear before the judgment-seat of Christ according to the words of the Apostle Paul, the Lord God has raised us from the dead and commanded us to make this statement to you. See to it that you are not deceived and excluded from the kingdom of God." When the Emperor Theodosius heard this he praised the Lord for not permitting His people to perish. But the men again lay down on the ground and fell asleep. The Emperor Theodosius wanted to make graves of gold for them, but in a vision he was prohibited from doing this. And until this very day these men rest in the same place, wrapped in fine linen mantles.

 
At the first glance there is nothing which betrays the Germanic origin of this legend. It may seemingly have had an independent origin anywhere in the Christian world, and particularly in the vicinity of Ephesus.
 
However the historian of the Franks, Bishop Gregory of Tours (born 538 or 539), is the first one who presented in writing the legend regarding the seven sleepers.[4] In the form given above it appears through him for the first time within the borders of the christianized western Europe (see Gregorius' Miraculorum Liber, I., ch. 92). After him it reappears in Greek records, and thence it travels on and finally gets to Arabia and Abyssinia.[5] His account is not written before the year 571 or 572. As the legend itself claims in its preserved form not to be older than the first years of the reign of Theodosius, it must have originated between the year's 379-572.
 
The next time we learn anything about the seven sleepers in occidental literature is in the Longobardian historian Paulus Diaconus (born about 723).[6] What he relates has greatly surprised investigators; for although he certainly was acquainted with the Christian version in regard to the seven men who sleep for generations in a cave, and although he entertained no doubt as to its truth, he nevertheless relates another - and that a Germanic - seven sleepers' legend, the scene of which is the remotest part of Germania. He narrates (I. 4):
 

"As my pen is still occupied with Germany, I deem it proper, in connection with some other miracles, to mention one which there is on the lips of everybody. In the remotest western boundaries of Germany is to be seen near the sea-strand under a high rock a cave where seven men have been sleeping no one knows how long. They are in the deepest sleep and uninfluenced by time, not only as to their bodies but also as to their garments, so that they are held in great honor by the savage and ignorant people, since time for so many years has left no trace either on their bodies or on their clothes. To judge from their dress they must be Romans. When a man from curiosity tried to undress one of them, it is said that his arm at once withered, and this punishment spread such a terror that nobody has since then dared to touch them. Doubtless it will some day be apparent why Divine Providence has so long preserved them. Perhaps by their preaching - for they are believed to be none other than Christians -- this people shall once more be called to salvation. In the vicinity of this place dwell the race of the Skritobinians ('the Skridfinns')."[7]

 In chapter 6 Paulus makes the following additions, which will be found to be of importance to our theme: "Not far from that sea-strand which I mentioned as lying far to the west (in the most remote Germany), where the boundless ocean extends, is found the unfathomably deep eddy which we traditionally call the navel of the sea. Twice a day it swallows the waves, and twice it vomits them forth again. Often, we are assured, ships are drawn into this eddy so violently that they look like arrows flying through the air, and frequently they perish in this abyss. But sometimes, when they are on the point of being swallowed up, they are driven back with the same terrible swiftness."
 
From what Paulus Diaconus here relates we learn that in the eighth century the common belief prevailed among the heathen Teutons that in the neighborhood of that ocean-maelstrom, caused by Hvergelmir ("the roaring kettle"), seven men slept from time immemorial under a rock. How far the heathen Teutons believed that these men were Romans and Christians, or whether this feature is to be attributed to a conjecture by Christian Teutons, and came through influence from the Christian version of the legend of the seven sleepers, is a question which it is not necessary to discuss at present. That they are some day to awake to preach Christianity to "the stubborn," still heathen Germanic tribes is manifestly a supposition on the part of Paulus himself, and he does not present it as anything else. It has nothing to do with the saga in its heathen form.
 
The first question now is: Has the heathen tradition in regard to the seven sleepers, which, according to the testimony of the Longobardian historian, was common among the heathen Teutons of the eighth century, since then disappeared without leaving any traces in our mythic records?
 
The answer is: Traces of it reappear in Saxo, in Adam of Bremen, in Norse and German popular belief, and in Völuspá. When compared with one another these traces are sufficient to determine the character and original place of the tradition in the epic of the Germanic mythology.
 
In  the main features of Saxo's account of King Gorm's and Thorkil's journey to and in the lower world in Book 8 of his Danish History, they and their companions are permitted to visit the abodes of torture of the damned and the fields of bliss, together with the gold-clad world-fountains, and to see the treasures preserved in their vicinity. In the same realm where these fountains are found there is, says Saxo, a tabernaculum within which still more precious treasures are preserved. It is an uberioris thesauri secretarium.[8] The Danish adventurers also entered here. The treasury was also an armory, and contained weapons suited to be borne by warriors of superhuman size. The owners and makers of these arms were also there, but they were perfectly quiet and as immovable as lifeless figures. Still they were not dead, but made the impression of being half-dead (semineces). By the enticing beauty and value of the treasures, and partly, too, by the dormant condition of the owners, the Danes were betrayed into an attempt to secure some of these precious things. Even the usually cautious Thorkil set a bad example and put his hand on a garment (amiculo manum inserens). We are not told by Saxo whether the garment covered anyone of those sleeping in the treasury, nor is it directly stated that the touching with the hand produced any disagreeable consequences for Thorkil. But further on Saxo relates that Thorkil became unrecognizable, because a withering or emaciation (marcor) had changed his body and the features of his face.

Leika Míms synir, en mjötuður kyndist að inu gamla Gjallarhorni; hátt blæs Heimdallur, horn er á lofti.  mælir Óðinn við Míms höfuð.

Mim’s sons play, but the fate tree kindles, at the ancient Gjallar-horn. Hard blows Heimdall, his horn is aloft; Odin speaks with Mim’s head.

  
In regard to leika, it is to be remembered that its old meaning, "to jump," "to leap," "to fly up," reappears not only in Ulfilas, who translates skirtan of the New Testament with laikan. (Luke I. 41, 44, and VI. 23; in the former passage in reference to the child slumbering in Elizabeth's womb; the child "leaps" at her meeting with Mary), but also in another passage in Völuspá, where it is said in regard to Ragnarok, leikur hár hiti við himin sjálfan -- "high leaps" (plays) "the fire against heaven itself." Further, we must point out the preterit form kyndisk (from kynna, to make known) by the side of the present form leika. This juxtaposition indicates that the sons of Mimir "rush up," while the fate of the world, the final destiny of creation in advance and immediately beforehand, was proclaimed "by the old Gjallarhorn." The bounding up of Mimir's sons is the effect of the first powerful blast. One or more of these follow: "Loud blows Heimdall -- the horn is raised; and Odin speaks with Mimir's head." Thus we have found the meaning of leika Míms synir. Their waking and appearance is one of the signs best remembered in the chronicles in popular traditions of Ragnarok's approach and the return of the dead, and in this strophe Völuspá has preserved the memory of the "sleeping castle" of Germanic mythology.
 
Thus a comparison of the mythic fragments extant with the popular traditions gives us the following outline of the Germanic myth concerning the seven sleepers:
 
The world-tree -- the representative of the physical and moral laws of the world -- grew in time's morning gloriously out of the fields of the three world-fountains, and during the first epochs of the mythological events (ár alda) it stood fresh and green, cared for by the subterranean guardians of these fountains. But the times became worse. The feminine counterpart of Loki, Gullveig-Heid, spreads evil runes in Asgard and Midgard, and he and she cause disputes and war between those god-clans whose task it is to watch over and sustain the order of the world in harmony. In the feud between the Aesir and Vanir, the middle and most important world-fountain -- the fountain of wisdom, the one from which the good runes were fetched -- became robbed of its watchman. Mimir was slain, and his seven sons, the superintendents of the seven seasons, who saw to it that these season-changes followed each other within the limits prescribed by the world-laws, were put to sleep, and fell into a stupor, which continues throughout the historical time until Ragnarok. Consquently the world-tree cannot help withering and growing old during the historical age. Still it is not to perish. Neither fire nor sword can harm it; and when evil has reached its climax, and when the present world is ended in the Ragnarok conflict and in Surt's flames, then it is to regain that freshness and splendor which it had in time's morning.
 
Until that time Sindri-Dvalin and Mimir's six other sons slumber in that golden hall which stands toward the north in the lower world, on Mimir's fields. Night, their sister, dwells in the same region, and shrouds the chambers of those slumbering in darkness. Standing toward the north beneath the Nida mountains, the hall is near Hvergelmir's fountain, which causes the famous maelstrom. As sons of Mimir, the great smith of antiquity, the seven brothers were themselves great smiths of antiquity, who, during the first happy epoch, gave to the gods and to nature the most beautiful treasures (Mjölnir, Brisingamen, Slidrugtanni,[17] Draupnir). The hall where they now rest is also a treasure-chamber, which preserves a number of splendid products of their skill as smiths, and among these are weapons, too large to be wielded by human hands, but intended to be employed by the brothers themselves when Ragnarok is at hand and the great decisive conflict comes between the powers of good and of evil. The seven sleepers are there clad in splendid mantles of another cut than those common among men. Certain mortals have had the privilege of seeing the realms of the lower world and of inspecting the hall where the seven brothers have their abode. But whoever ventured to touch their treasures, or was allured by the splendor of their mantles to attempt to secure any of them, was punished by the drooping and withering of his limbs.
When Ragnarok is at hand, the aged and abused world-tree trembles, and Heimdall's trumpet, until then kept in the deepest shade of the tree, is once more in the hand of the god, and at a world-piercing blast from this trumpet Mimir's seven sons start up from their sleep and arm themselves to take part in the last conflict. This is to end with the victory of the good; the world-tree will grow green again and flourish under the care of its former keepers; "all evil shall then cease, and Baldur shall come back." The Germanic myth in regard to the seven sleepers is thus most intimately connected with the myth concerning the return of the dead Baldur and of the other dead men from the lower world, with the idea of resurrection and the regeneration of the world. It forms an integral part of the great epic of Germanic mythology, and could not be spared. If the world-tree is to age during the historical epoch, and if the present period of time is to progress toward ruin, then this must have its epic cause in the fact that the keepers of the chief root of the tree were severed by the course of events from their important occupation. Therefore Mimir dies; therefore his sons sink into the sleep of ages. But it is necessary that they should wake and resume their occupation, for there is to be a regeneration, and the world-tree is to bloom with new freshness.

Both in Germany and in Sweden there still prevails a popular belief which puts "the seven sleepers" in connection with the weather. If it rains on the day of the seven sleepers, then, according to this popular belief, it is to rain for seven weeks thereafter. People have wondered how a weather prophecy could be connected with the sleeping saints, and the matter would also, in reality, be utterly incomprehensible if the legend were of Christian origin; but it is satisfactorily explained by the heathen-Germanic mythology, where the seven sleepers represent those very seven so-called economic months - the seven changes of the weather - which gave rise to the division of the year into the months - gormánuðr, frerm., hrútm., einm., sólm., selm., and kornskurðarmánuðr. Navigation was also believed to be under the protection of the seven sleepers, and this we can understand when we remember that the hall of Mimir's sons was thought to stand near the Hvergelmir fountain and the Grotti of the skerry, "dangerous to seamen," and that they, like their father, were lovers of men. Thorkil, the great navigator of the saga, therefore praises Gudmund-Mimir as a protector in dangers.[18]

The legend has preserved the connection found in the myth between the above meaning and the idea of a resurrection of the dead. But in the myth concerning Mimir's seven sons this idea is most intimately connected with the myth itself, and is, with epic logic, united with the whole mythological system. In the legend, on the other hand, the resurrection idea is put on as a trade-mark. The seven men in Ephesus are lulled into their long sleep, and are waked again to appear before Theodosius, the emperor, to preach a sermon illustrated by their own fate against the false doctrine which tries to deny the resurrection of the dead.

Gregorius says that he is the first who recorded in the Latin language this miracle, not before known to the Church of Western Europe. As his authority he quotes "a certain Syrian" who had interpreted the story for him. There was also need of a man from the Orient as an authority when a previously unknown miracle was to be presented -- a miracle that had transpired in a cave near Ephesus. But there is no absolute reason for assuming that Gregorius presents a story of his own invention.  The reference of the legend to Ephesus is explained by the antique saga-variation concerning Endymion,[19] according to which the latter was sentenced to confinement and eternal sleep in a cave in the mountain Latmos. Latmos is south of Ephesus, and not very far from there. This saga is the antique root-thread of the legend, out of which rose its localization, but not its contents and its details. The contents are borrowed from the Germanic mythology. That Syria or Asia Minor was the scene of its transformation into a Christian legend is possible, and is not surprising. During and immediately after the time to which the legend itself refers the resurrection of the seven sleepers, the time of Theodosius, the Roman Orient, Asia Minor, Syria, and Egypt were full of Germanic warriors who had permanent quarters there. A Notitia dignitatum[20] from this age speaks of hosts of Goths, Alamannians, Franks, Chamavians, and Vandals, who there had fixed military quarters. There then stood an ala Francorum, a cohors Alamannorum, a cohors Chamavorum, an ala Vandilorum, a cohors Gothorum,[21] and no doubt there, as elsewhere in the Roman Empire, great provinces were colonized by Germanic veterans and other immigrants. Nor must we neglect to remark that the legend refers the falling asleep of the seven men to the time of Decius. Decius fell in battle against the Goths, who, a few years later, invaded Asia Minor and captured among other places also Ephesus. [22]