People can become dependent on sleep medicines, which can slow them down and make them feel even more depressed — and, especially in older adults, impair memory abilities.

Researchers at the University of Pennsylvania performed sleep studies on volunteers who were assigned to four groups: those who were allowed to sleep up to eight hours, six hours, or four hours each night for a two-week period, and one group that was deprived of all sleep for three days in a row.

They found significant memory deficits in the groups that slept for only four to six hours each night for two weeks.

Their memory impairments were similar to those in the group who went without any sleep at all for three consecutive days.

While the volunteers sleeping four to six hours felt only slightly sleepy in the day, they were unaware of the extent of their cognitive impairments resulting from sleep deprivation.

The good news is that when our sleep patterns and quality improve, so do our memory abilities.

Sleep medications can also suppress dreaming, or what’s known as rapid eye movement (REM) sleep. When people stop taking medicines, many of them report very vivid dreams — a phenomenon doctors refer to as “REM rebound.”

What is actually happening is that dreams that were suppressed by the medicine finally have a chance to be released.

Many of us assume that sleep is just a passive, resting state. And of course, by definition sleep is a time of rest.

But our brains are actually very active during sleep — especially during periods of REM sleep.

While we experience REM sleep, we not only dream but our eyes and eyelids flutter and our breathing becomes irregular.

REM sleep is a normal and necessary brain activity that maintains neural health.

Recent research has shown that during sleep our brains are also getting rid of toxins. Dr. Maiken Nedergaard of the University of Rochester reported that in animals, cerebral spinal fluid volume increases during sleep, and this fluid clears toxins from the brain.

When the animals awaken, their brain cells enlarge and the flow of their spinal fluid diminishes, which may explain why we feel clear-headed after a good night’s sleep — toxins have been removed from our brains.