Some of the common causes of sleeplessness are as follows:
Inappropriate sleeping environment such as: too hot or too cold bedroom, bed being too hard or too soft, noisy place, poor ventilation, change of sleeping place, mosquitoes, etc
Anxiety: Worrying about various things such as work, appointments, family or work stress, etc.
Depression, grief, etc
Emotional excitement (Euphoria)
Mood disorders
Too much of caffeine intake especially during the later part of the day (e.g. Strong coffee, cola, etc).
Sedentary lifestyle: When you have not worked enough during the daytime, your mind might be tired, but your body may not be tired, thus causing difficulty in sleeping
Hormonal changes: Around the time of menses, Pregnancy (especially during 1st trimester)
Overeating
Jet lag
Working in shifts
Systemic disorders such as Asthma, Hyperthyroidism, duodenal ulcers, arthritis, Diabetes, GERD, etc
Night urination frequency (due to diabetes, prostate enlargement, urinary infection, etc)
Alcohol withdrawal
Dehydration
Chronic pain (due to any cause): E.g.: Osteoarthritis, trauma, leg cramps, etc
Itching (due to any skin disorder, allergy, etc)
There are a large number of drugs that can causes sleeplessness. Few of them have been mentioned below. Please note however that this list is not exhaustive:
