There are several stages of sleeping which repeat themselves in a cycle several times during the night. You won’t be able to notice them when you’re asleep. This includes periods of light sleeping (where you’re easily awoken), and deep sleeping (which is harder to wake up from) and which is where dreaming occurs.
As adults get older, their sleep patterns and circadian rhythm begin to change. The circadian rhythm controls what time the body decides to sleep. If you live with your grandparents, you might wake up to hear them already walking around the house. As people age, their circadian rhythm is normally shifted forward and they tend to sleep earlier at night and wake up earlier in the day. They also don’t sleep as deeply, spending more time in the period of light sleeping than deep sleeping and so are woken up easier.
This is one of the reasons why some elderly people nap during the day. They constantly wake up in the night, making them extremely tired in the day time. Naps are then used to try to catch up on the amount of lost sleep from the night before.
However, sometimes older adults can’t seem to sleep because of other non-natural problems.
Either way, a lack of sleep usually makes the person easily annoyed or makes it harder for them to concentrate during the day.