Sleep structure is qualitatively and quantitatively changed by aging. The elderly usually go to bed in early evening and wake up in early morning, and they also take several naps in the day time. The polyphasic sleep is one of the typical sleep patterns found in the elderly. Comparing the sleep of the elderly with that of young adults by the method of polysomnography, the characteristics of the sleep of the elderly are in the prolongation of sleep latency, shortening of total sleep time, increase of Stage W and Stage 1, decrease of Stage 3 and 4, and also decrease of Stage REM and the advance of REM phase. Insomnia is a frequently observed symptom in the elderly. The so-called psychophysiological insomnia due to transient psychological or situational stress is common in the elderly. However, insomnia following the mental disturbance (depression), chronic use of drug or alcohol, dementia (vascular or Alzheimer type) are also important in the elderly. Sleep apnea syndrome is recently found as an important cause of insomnia. Concerning the treatment and prevention of insomnia, it is necessary to exclude the causes of insomnia, to improve the environmental conditions and to keep the regular rhythm of sleep-wake cycle. It is also important to carefully select and use the adequate hypnotics considering the pharmacokinetics and adverse effects of the drugs in the elderly.