Well the best way to describe the Odinsleep is, Think of Odin a rechargeable battery, every now and then he needs to recharge his godly energy called the Odin Force (kind of like Silver Surfer's Power Cosmic but a magical version). It flows through EVERYTHING and Everyone on Asgard.
Now depending how active he is using the Odin Force depends on how long he is in an Odinsleep.
In Marvel's mythology, if Odin has an "average/normal" year, Marvel's Odin goes into a yearly Odinsleep during winter. If he's highly active, or something drains alot of the Odin Force from him he would need to recharge sooner.
While IN the Odinsleep, he's actually between Life and Death. He had no heart beat during this period. He can be considered as being in a godly Coma, he's dead, but, not dead.
As for Loki, the Frost Giants in the Comics and Myth are Shape Shifters.
In the Myth one time, there was a Frost giant disguised as a Asgardian Brick layer, and the wall around Asgard was damaged during a battle. Long story Short, Loki ends up turning into a female horse to distract the Brick layers male horse. The male horse ends up getting Horse-Loki pregnant, and Loki gives birth to Odin's Horse Slepnir.
In the Movie, I think Odin taps that natural power to hide Loki's true form when Odin finds the abandoned Loki.