4. You're up all night.
The day is done, but your mind's still buzzing -- leaving you to toss and turn in bed. You aren't the only one lying there wide awake. "Stress is the number-one cause of sleep deprivation and insomnia," says James Maas, Ph.D., author of Power Sleep. "That lack of shut-eye, in turn, makes you irritable and anxious, decreases your ability to fight infection, and impairs your ability to concentrate." Sleep problems also create a vicious circle: Once you're sleep deprived, your threshold for dealing with everyday problems is even lower -- keeping you stressed out and making it more difficult to sleep the following night.