DEAR DOCTOR K:
I read about a new study on sleep and memory. I understand why too little sleep could affect memory. But why would too much sleep be harmful?
When it comes to memory, sleep is a Goldilocks issue: Neither too much nor too little is good.Aim for “just right,” says Dr. Elizabeth Devore, an instructor in medicine at Harvard-affiliated Brigham and Women’s Hospital. Dr. Devore led a new study that suggests getting an “average” amount of sleep — seven hours per day — may help maintain memory in later life.
In 1986 and again in 2000, researchers asked a large number of women who were enrolled in a long-term study about their sleep habits. Women who slept five hours or fewer per night or nine hours or more performed worse on brain testing compared with women who slept seven to eight hours a night. The researchers estimated that “undersleepers” and “oversleepers” were mentally two years older than the women who got seven to eight hours of shut-eye a night.
A study like this one can find only an association, a connection, between two things — in this case, the number of hours of sleep and memory. It cannot prove that sleeping more or less than seven or eight hours a night (on a regular basis) is bad for memory. It can only suggest that possibility.
Is the possibility plausible? If so, how might sleep affect memory? People who are persistently sleep-deprived are more likely to have conditions that can decrease blood flow inside the brain. The most common is atherosclerosis of the arteries of the brain. Atherosclerosis blocks arteries, and thereby slows blood flow. This may negatively affect the ability of brain cells to work properly.
Sleep-deprived brains may also collect more deposits of a protein called beta amyloid. These deposits are linked to declines in memory and thinking and increased risk of dementia.
One of the most interesting and potentially important scientific reports of the past year was a study done in mice. It may, or may not, apply to humans. The study showed that during sleep the brain “flushes out” toxic substances. If this is true in humans, it could be part of the explanation as to why sleeping fewer than seven or eight hours per night on a regular basis could impair memory. But it wouldn’t explain why people who regularly sleep more than seven or eight hours per night have impaired memory.
What might the explanation be? People who spend more than nine or 10 hours a night in bed often have poor sleep quality. So for both too little and too much sleep, the important number may be the hours of quality sleep.
To improve your sleep:
- Go to bed and wake up at the same time every day.
- Use your bed only for sleeping or lovemaking.
- If you can’t fall asleep after 15 to 20 minutes, get out of bed. Get back into bed only when you feel sleepy.
- Limit caffeine.
- Don’t take long naps during the day.