A friend of mine cannot sleep, even after lying on the bed for a long time. Lets call this person S. S can lie on the bed at 12am, but sleeps only at 2am. So I have decided to research on this problem.
This might a sleeping disorder, and not necessary insomnia, as what we usually call it when somebody cannot fall asleep. Insomnia is a symptom of sleeping disorder, and not a stand alone diagnose or a disease. According to Wikipedia, sleeping disorder (somnipathy) is a medical disorder of the sleep patterns of a person or animal.
But I shall focus on insomnia, as I think this is more common. By definition, insomnia is "difficulty initiating or maintaining sleep, or both".

Causes of Insomnia
There is three main causes of insomnia, and they are namely, psychological, physical and temporary events or factors.

• Psychological Causes
• Anxiety, condition which one feel increased tension, apprehensions and feelings of helplessness, worry, fear and uncertainty.
• Stress, or how effectively a person copes with any emotional, physical, social, economic, or other factor that requires a response or change.
• Depression, a mood disturbance characterized by feelings of sadness, despair and discouragement
• In addition, a lack of good nights sleep can lead to these psychological problems, and a vicious cycle can develop.
• Physical Causes
  
• Medical conditions such as heart disease, high blood pressure and Parkinson's disease.
  
• Pain and discomfort from a medical illness or injury often interfere with sleep.
• Genetics. Problems with insomnia do seem to run in some families, although researchers have yet to identify how genetics play a role.
• Other sleep disorders. These include sleep apnea(in which one temporarily stops breathing during sleep) and periodic leg and arm movements during sleep (in which one's muscles excessively twitch or jerk).
• Temporary events or Factors
• Jet lag. Air travel across time zones often causes brief bouts of insomnia.
• Working the night shift or long shifts. Individuals who work at night and those who work long shifts may have trouble adjusting their sleep habits.
• Medications. Insomnia can be a side effect of various medications, both prescription and over-the-counter.
• Overuse of caffeine and alcohol. Caffeine most commonly disrupts sleep. While a drink or two before bed may help a person relax, more than that can lead to fragmented sleep and wakefulness a few hours later.
• Environmental noise, extreme temperatures, or a change in a person's surrounding environment.

Consequences of Insomnia

• It can affect concentration and memory, which may affect one's ability to perform daily tasks.
  
• Insomnia contributes to traffic and industrial accidents. Various studies have shown that fatigue plays a major role in automobile and machinery accidents. As many as 100,000 automobile accidents, accounting for 1,500 deaths, are caused by sleepiness.
  
• Insomnia increases the activity of hormones and pathways in the brain that causes stress, and changes in sleeping patterns have shown to affect moods.
  
• A study reported that people who had chronic insomnia had symptoms of heart and nervous system activity that might put them at risk for heart disease.
  
• Headaches which occured during the night or early morning might be related to sleep disorder.
  

Cure for Insomnia

• See a Doctor

• Insomnia can be a symptom of physical disorders, although for most of us it's the result of tension, stress and anxiety—and of course the more anxious we get about our insomnia, the worse it gets. If your doctor pronounces you a "healthy" insomniac, he might suggest some of the techniques provided here. Or she might prescribe drugs to help you get to sleep.

• We suggest you try all these methods first, and use drugs only as a last resort. The decision, of course, is yours.

• Take a Warm Bath

  It's a great way to relax your body.

• Get a Massage

  Have somebody to give you a massage just before going to sleep. Have them make the massage strokes slow, gentle, yet firm, to work the tension out of your muscles and soothe you to sleep.

• Listen to Music

  Play some soft, soothing music that will lull you to sleep. There are even cassettes and records designed for that very purpose. Some are specially composed music, others simply have sounds of waves rhythmically breaking, or the steady pattern of a heartbeat.

• Drink Warm Milk

  A glass of warm milk 15 minutes before going to bed will soothe your nervous system. Milk contains calcium, which works directly on jagged nerves to make them (and you) relax.

Lets hope that S will be able to sleep well soon. :)

Credits

http://www.ehealthmd.com/library/insomnia/INS_causes.html

http://en.wikipedia.org/wiki/Insomnia

http://www.ehealthmd.com/library/insomnia/ins_whatis.html

http://www.well.com/~mick/insomnia/

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