Nicotine: is a stimulant, so smoking in the evening is likely to make it more difficult for us to go to sleep.
Caffeine: Our brain cells produce adenosine, a chemical that serves as an off switch to slow down brain activity. Coffees, colas, chocolate, and other caffeinated substance blocks adenosine receptors, thus overriding the brains off switch for six to eight hours.
Alcohol: Taking alcohol to help induce sleep is a bad idea, Alcohol may help us fall asleep, but it results in light, fragmented sleep. After the sedative effect wears off (usually in 4 hours) there's a rebound effect that can leave you wide awake in the middle of the night.
Food or exercise too close to bedtime: Both boost the body's metabolism which can drive away sleep.
Irregular schedules: Mostly shifting workers experience this. Abruptly changing bedtimes disrupts the body's cycle. The result were wide awake when we really should be asleep and drowsy when were supposed to be alert.
Depression: People with the blues often wake up early and cant sleep again, a condition called sleep maintenance insomnia.
Stress: The most probably common cause of sleepless nights. It increases heart rate and blood pressure, raising arousal levels.
Check yourself maybe you fall with one or more in this list, no one can help you sleep soundly, only you can help yourself.