Three main factors affect our ability to sleep. First, our need for sleep is similar to our need for food or water: the longer we go without it the more we need it, and the easier it is to fall asleep. This is why “sleeping in” or napping makes it harder to get to sleep at bedtime the next night.
Secondly, our body has an internal clock called the circadian rhythm, which controls sleepiness and wakefulness. It is much easier to enter sleep at certain times of day, especially later in the evening if you work the day shift. Other factors affecting these internal clocks are air travel going east or west (jet lag), inconsistent sleep and wake times or changes in the time for sunrise or sunset.
Thirdly, a state of increased mental arousal will prevent sleep. At times this arousal is helpful such as when a danger is perceived. But when this increased attention is chronic and not related to real danger, it can interfere with sleep. Those who suffer from this perceive themselves as anxious.
An increased state of arousal and disturbed sleep may also be caused by some medications and drugs, such as caffeine, nicotine, cocaine, amphetamines, decongestants, diet pills, some steroid medicines, and certain anti-depressant medicines. Some people with chronic insomnia have learned to dread bedtime because of the nightly struggle to get sleep, so when bedtime approaches, their level of mental arousal actually goes up and interferes with their ability to sleep.
There are other factors that perpetuate insomnia, which include: worrying about not falling asleep, caffeine, alcohol, smoking, excessive napping and irregular or disrupted sleep schedules. Sometimes stopping these behaviors may eliminate insomnia all together.