The importance of exposure to day light and artificial light in altering a person's internal clock cannot be over stated. For example people with chronic pain are almost invariably poor sleepers. The process of falling asleep requires complete relaxation that may be impossible in the presence of significant pain or psychological disturbance. Treating the cause of the pain is the most effective approach. Insomnia is a frequent presenting complaint for depression and specific questioning will disclose this condition.
Sleep apnoea is the commonest form of daytime sleepiness and poor night time sleep.
After excluding organic and psychological causes for insomnia, we are left with a group that is most likely to have a large behavioural component. Shift workers are in a special group that usually have chronic sleep deprivation, a recurrent disruption to sleep pattern and an inability due to work commitments to alter the situation. It is well recognized that shift workers have inferior health to people working permanent day shift. Jet-lag is similar to the sleep problems experienced by shift workers. Usually the body adjusts to the new time zone at about 1 hour a day.