"Sleep that knits up the ravelled sleave of care... Chief nourisher in life's feast."
Shakespeare knew how important a good night's sleep was.

So do we, but usually it takes a night or two of poor sleep before we come to realize it fully.

When we are young, we can often race around all day, and when night comes, sleep like the dead, waking in the morning refreshed and alive - ready for another day filled with activity. As we age this changes.

As we get on in years, we find we are less able to sleep through the night.
And when we awaken, have greater difficulty falling back to sleep.

Not only do we suffer more broken sleep as we get older, the quality of our sleep declines. We tend to sleep less deeply. Also, we tend to achieve less time in REM sleep, the sleep during which we dream.

REM sleep is acutely important to our mental health.
One experiment in psychology showed that where experimental subjects were deprived of REM sleep (but were allowed to get enough sleep otherwise), they began to exhibit symptoms of mental illness, and for some, these symptoms extended even to the point where they began to hallucinate. When they were allowed to catch up on their REM sleep, these symptoms disappeared.