What's said about spell save is valid, but this is as well. Assistance from other players can take many forms, including, but not limited to, waking your slept character, transporting him/her away from your assailant, providing other support spell aid, or simply killing or driving off your assailant(s) so the sleep spell is a moot point. Obviously, not all of these will be available to all characters in all situations.

More exotic defenses against sleep are also possible, ranging from utilizing creature tracking to your advantage to simply ensuring your enemy with the spell never catches you both visible and in the same room with them.