Sleep and cognitive health

A bright airy room at dawn with soft light falling through linen curtains onto a neatly folded blanket

Poor sleep affects memory immediately and. Anyone who has tried to hold a conversation after a bad night knows this without being told.

The longer-term relationship between sleep and dementia risk is genuinely complicated, and worth being careful about.

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Cause, effect, or both

Disturbed sleep is common in people who go on to develop dementia. That does not establish which way the arrow points.

Poor sleep may contribute to risk, changes in the brain may disturb sleep before anything else is noticeable, or both may be true at once. Anyone telling you the answer is settled is overstating it.

What is not in doubt is that poor sleep degrades memory performance now, which matters if you are trying to judge whether something has changed.

When to get it looked at

some sleep problems are treatable and worth pursuing for their own sake.

  • Loud snoring with pauses in breathing. Sleep apnoea is common, treatable, and affects daytime cognition.
  • Acting out dreams physically. Worth mentioning to a doctor.
  • Persistent insomnia. Treatable, and worth treating.
  • New daytime sleepiness. Particularly if it has changed recently.

Also worth reading: sleep's role in memory protection.

A Black woman in her seventies watching the street wake up from a kitchen window
A white man in his seventies folding laundry warm from the line

How much sleep

for most adults the answer is seven to nine hours, and the relationship with cognition appears to be a curve rather than a line: both short and unusually long sleep are associated with worse outcomes.

Long sleep is probably not the cause. Needing ten hours is more often a symptom of something else, including depression, sleep apnoea, thyroid problems or medication, all of which are worth investigating in their own right.

Older adults often sleep in a more fragmented way and wake earlier, and that shift is a normal part of ageing rather than a warning sign on its own.

A Latina woman in her sixties switching off a bedside lamp, book face down

What to try before anything else

  • a consistent wake time. More effective than a consistent bedtime, and easier to control.
  • Daylight early. Morning light outdoors anchors the body clock better than any indoor lighting.
  • Caffeine before noon. Its half-life is around five hours, and longer in older adults.
  • Alcohol is not a sleep aid. It shortens the time to fall asleep and degrades the second half of the night.
  • Get up if you are awake. Lying in bed frustrated teaches your brain that bed is where you lie awake.

If those do not help within a few weeks, that is a reason to see a doctor rather than to keep trying harder. Insomnia has effective treatments, and cognitive behavioural therapy for insomnia outperforms sleeping tablets over the long run.

A man in his fifties and his mother in her late seventies sit

If you are taking a memory test

Take it when you are reasonably rested.

Memory performance moves around with sleep, illness and stress. If you are tracking over time, testing under similar conditions each time is what separates a trend from the weather.

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