If your sleep trouble is confined to difficulty falling asleep, the time you are choosing to go to sleep may not be synchronized with your biological clock. The biological processes that initiate and maintain sleep in humans are active throughout the night. Opposing this sleep tendency, however, is the alerting action of the biological clock that is active throughout the day. When the biological clock is active at your scheduled bedtime, you will have sleep-onset insomnia.
The prevalence of insomnia is higher among older people and women. Women suffer loss of sleep in connection with menstruation, pregnancy, and menopause. Rates of insomnia increase as a function of age but most often the sleep disturbance is attributable to some other medical condition.