Sleep is a fairly common status affliction and is most certainly the most self-inflicted, due to the HP recovery move Rest. If a Pokémon uses Rest it will sleep only 2 turns, whereas if an opponent puts a Pokémon to sleep the afflicted may be asleep anywhere from 1 to 4 turns. When put to sleep by an opponent the afflicted does not regain HP as it would if it used Rest.
Pokémon using physical attacks against a Pokémon that has the ability Effect Spore may be put sleep at a rate of 10% (1/10).
When used on a Pokémon that is asleep the move Wake-Up Slap will awaken the target.
A Pokémon with Early Bird may remove this status at a rate of 50% (1/2) before it attacks each turn.
A Pokémon that is asleep when a foe on the field has the Bad Dreams ability that ability will activate and deal damage to the sleeping Pokémon until it wakes up. The moves Dream Eater Nightmare are also now able to work.
Vital Spirit prevents this affliction altogether whereas the ability Leaf Guard when the weather is Strong Sunlight. Natural Cure will remove this status effect when switching out of battle. Hydration will remove the status, after being inflicted, when the weather is Heavy Rain.