One third of our lives have been spent on sleeping, but sometimes sleep can be elusive. 

Everyone almost have had transient insomnia. Insomnia is the inability to fall asleep once in a while, or feeling tired everytime you wake up.

Insomnia that is chronic, lasts longer.  Medical or psychiatric cause are the common causes of insomnia.  Others may be brought by 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.
 
Good sleep habits offered by Duke University:

*  When you get up, try to keep a schedule like, eat, take medicine and go to bed, etc.
*  Try to establish a relaxing before-sleep ritual, like taking warm bath, having light snack or reading for 10 minutes.  This can let your body become informed that it is already near bed time.
*  When sleepy try to go to bed, get out of bed when not sleeping.  
*  Have a regular exercise, it is best to exercise in the late afternoon, about six hours before bedtime.
*  Around bedtime, try to avoid caffeine, alcohol and smoking.
*  Don't nap, naps are best taken in the mid-afternoon.

More assesment and treatment may be needed if these habits are not working, immediately tell your doctor.