(HealthDay News) -- We spend about one-third of our lives asleep, but sometimes sleep can be elusive.

Almost everyone has had transient insomnia -- the inability to fall asleep once in a while, or waking up feeling tired.

Chronic insomnia lasts longer. The common condition may be brought on by medical or psychiatric causes, such as colds, pain or depression. But 10 percent of all insomnia occurs in the absence of any medical or psychiatric disorder, and is called primary insomnia.

Duke University offers these good sleep habits:

• Keep a schedule for when you get up, eat, take medicines, go to bed, etc.
• Establish a relaxing pre-sleep ritual, such as taking a warm bath, reading for 10 minutes, or having a light snack. This lets your body know that bedtime is near.
• Go to bed only when sleepy and get out of bed if you're not sleeping. By spending long periods awake in bed, your body learns that it's OK to be awake in bed.
• Exercise regularly. It's best to exercise in the late afternoon, about six hours before bedtime.
• Avoid caffeine, alcohol and smoking around bedtime.
• Don't nap. If you must, it's best to take naps in mid-afternoon.

If these habits aren't working, tell your doctor. More assessment and treatment may be needed.