Once a year the king of Asgard, Odin must enter a deep sleep to renew his godly powers (Odinforce) and extend his life. If Odin exhausts his powers before the year is up or sustains serious injury he is sometimes forced to enter the Odinsleep early to heal and restore his powers. If Odin does not enter it his godly powers weaken and he becomes very vulnerable. Waking Odin during this time is usually unwise due to the serious nature of it. His powers will not be fully restored should he be woken early and it could even threaten his life. The length of the Odinsleep is usually a day, but depending on Odin's state upon entering it can be much longer.
Asgard is especially vulnerable during this time without Odin's power to protect it and Odin's own vulnerability. Odin is always under heavy guard with his room warded by powerful magics. Asgard often finds itself under heavy attack during these periods and relies heavily on Thor for protection. Many times a crisis is averted only by Asgard's defenders holding out long enough for Odin to awaken and deal with the threat himself.
When in possession of the Odinpower Thor requires the Odinsleep as well under the same conditions as Odin. Odin is the only Norse god and skyfather that is known to require this form of renewal.