Why do my Sims keep getting out of bed without sleeping?
Are any other of their needs really low? If they're hungry or bored for example then that might interfere with them tying to sleep.
Are there any TV's, stereos, computers, etc. on in the same room they are trying to sleep in? I sometimes forget about this or don't realize that another Sim has come in and turned on the TV or radio and now my Sim won't sleep.