Common causes of insomnia

Lifestyle
Irregular hours due to excessive socialising
Jet lag
Alcohol/caffeine/drugs (Alcohol does sometimes help people sleep, but it is only a light sleep, not the deep restorative REM sleep.)
Insufficient exercise

Environment
Excessive noise (eg partner snoring or working in the next room, dog barking)
Uncomfortable mattress or pillow
Overheated/underheated room

Health problems
Pain from injury or illness
Indigestion (eating a heavy /oily meal could upset digestion)
Eczema/skin irritations
Asthma

Psychological
Stress (eg argument with partner, a sick child)
Grief (eg family member passed away, relationship breakdown)
Depression4

Medication/Drugs
Side effects of certain drugs (some cold/asthma medications) can cause or contribute to insomnia4
Heroine, amphetamines and ecstasy can interfere with sleep quality5

Sleep Disorders
Narcolepsy – An extreme tendency to fall asleep at any time without any control.
Restless Leg Syndrome- A disorder where an unpleasant prickling/tingling feeling is felt in the legs or feet, accompanied by a need to move them.4

Sleep Apnea
Irregular breathing during sleep