Physiology of Sleep

Sleep in different age groups

Sleep in adults

       In adults, sleep of 8-8.4 hours is considered fully restorative. In some cultures, total sleep often is divided into an overnight sleep period of 6-7 hours and a midafternoon nap of 1-2 hours.

       Stage I is considered a transition between wake and sleep. It occurs upon falling asleep and during brief arousal periods within sleep and usually accounts for 5-10% of total sleep time. Stage II occurs throughout the sleep period and represents 40-50% of total sleep time. Stages III and IV of sleep occur mostly in the first third of the night. They are distinguished from each other only by the percentage of delta activity and represent up to 20% of total sleep time. REM represents 20-25% of total sleep time.

Sleep in infants

       Infants have an overall greater total sleep time than any other age group; their sleep time can be divided into multiple periods. In newborns, the total sleep duration in a day can be 14-16 hours.

       Over the first several months of life, sleep time decreases; by age 5-6 months, sleep consolidates into an overnight period with at least 1 nap during the day. REM sleep in infants represents a larger percentage of the total sleep at the expense of stages III and IV. Until age 3-4 months, newborns transition from wake into REM sleep. Thereafter, wake begins to transition directly into NREM.

       Overall, electrocortical recorded voltage remains high during sleep, as it does during periods of wakefulness. Sleep spindles begin appearing in the second month of life with a density greater than that seen in adults. After the first year, the spindles begin decreasing in density and progress toward adult patterns. K complexes begin by the sixth month of life.

Sleep in the elderly

       In the elderly, the time spent in stages III and IV sleep decreases by 15% and the time in stage II increases by 5% compared to young adults, representing an overall decrease in total sleep duration. Latency to fall asleep and the number and duration of overnight arousal periods increase.This means the person need to sleep more to feel restored. If the elderly person does not increase the total time in bed, complaints of insomnia and chronic sleepiness may occur. Sleep fragmentation results from waking up in the middle of sleep and may be elicited by the increasing number of geriatric medical conditions, including sleep apnea, musculoskeletal disorders, and cardiopulmonary disease.