Periods of REM sleep last about 20 minutes and occur four or five times during the night. They alternate with longer periods of non-REM sleep, when the body functions slow down. As the night goes on, the periods of non-REM sleep become progressively lighter. Stage 4 sleep is felt to be deep and restorative, and it is during this time that the body repairs itself, utilizing a hormone called somatostatin. Deficient stage 4 sleep is felt to be an important factor in chronically painful conditions, such as fibromyalgia.
Sleeplessness is also common during pregnancy, especially in the later weeks.
People with asthma are unable to sleep if they are having breathing problems. Other medical problems can seriously interfere with the length and quality of sleep. For still others, the cause of insomnia may be attributed to their social environment. Vigorous exercise, late in the evening, can interfere with sleep onset. In the majority of cases, however, the core problem is emotional.