The human sleep cycle includes non-rapid eye movement (NREM) and rapid eye movement (REM). The non-rapid eye movement period is divided into light sleep and deep sleep, and a cycle is about 90 minutes.
Most of us sleep from non-rapid eye movement sleep, and enter the REM sleep period 90 minutes after falling asleep. The REM sleep period lasts for about 30 minutes.
Since then, the NREM and REM periods alternately appear, showing periodic changes. Our sleep is carried out in one cycle after another, and sleep will go through 4 to 5 cycles repeatedly every night.
The elderly have reduced deep sleep and increased shallow sleep, while becoming more sensitive to external environmental factors. I believe that every family with elderly people has heard the elderly complain about something that makes them unable to sleep. It is precisely because of external interference that they frequently wake up from light sleep, and ultimately reduce their overall sleep time.