Interrupted sleep harms memory
MATTHEW EDLUND M.D.
Imagine this — you want to wake up an animal so quickly and efficiently that you do not change overall sleep, just cause brief arousals that don’t shift the amount of REM, deep sleep or how long you get to rest. And you do it by changing the animal’s genetics using viruses as finely etched probes that help stimulate a tiny group of neurons with direct light, brought by cables so small they don’t materially change brain function, all the while monitoring with second-by-second precision.
It’s called optogenetics. The cells stimulated and monitored were for hypocretin-orexin, the neurotransmitter that disappears in narcolepsy and leaves narcoleptics falling uncontrollably asleep. And the reason the Stanford team under Craig Heller and Luis Licea did all this was to answer a question you can’t answer in people — if you wake up mice more than usual, but don’t change their overall sleep pattern, what happens?
The answer: You mess up their memory and learning with relatively few 10-second stimulations — even though the rest of their sleep remains “normal.”
The implication: wake people up enough times and you mess up their physiology, even if they can’t recall the awakenings and objectively sleep as much as any normal sleeper.
The clinical bottom line: keep people sleeping without arousals as much as you can to improve overall health and function.
Animals and people
Were these data unexpected? Somewhat, particularly given the very considerable technological sophistication they required. Michael Bonnet of the Wright State School of Medicine is one of many who studied what brief arousals did to human sleep.
The results he got were not pretty. Wake up “perfect” young sleepers for three seconds at a time often enough, and they felt like they were up all night — even if they slept 95 percent or more of the night.
Interrupt human sleep and you can’t get enough deep sleep and REM sleep. The beauty of the present study from Craig Heller’s lab was that the mice, generally interrupted at 60-120 second intervals, looked OK, had normal and unchanged sleep indices, showed no increases in steroid stress hormone levels and behaved pretty much the same — except that memory function was considerably diminished. All it took was decreasing the intervals between arousals about 30-40 percent above normal.
That’s because humans and animals wake up from sleep all the time. Yet we usually don’t know it. Unless people are up five to six minutes or more, they don’t remember awakening at all. My record: a woman who woke up 1,200 times through the night and thanked the sleep technicians for her “best sleep in four years.” She did not recall any awakenings.
Our ability to assess our ever-shifting consciousness is sometimes poor. We forget the last few minutes before we fall asleep. We fall asleep without knowing it — like the microsleeps we have driving. We wake up throughout the night without knowing it. And microarousals, as in this study and in many others done with people, can really diminish productivity, memory and learning.
How to sustain sleep
Fortunately, there’s quite a bit you can do to sustain sleep. The standard recommendation — have a sleep ritual for the hour before sleep where you do calming things to rest before sleep will often improve sleep quality and lead to less awakenings.
What is a sleep ritual? You floss your teeth; roll down the bed; put out your clothes for the next day and read a book you should have read in high school that takes you far, far away.
But there are specific things you can do that may decrease arousals through the night:
1. Turn off your electronics. Don’t text or look at e-mails during the night. The bright light from LED diodes may rapidly alter your melatonin production and lead you to be more aroused throughout the night.
2. Turn down the lights in the evening, especially the hour before sleep. There’s pretty good evidence that just increasing the amount of blue light in evening lamps will lead to more arousals during the night.
3. Take a hot bath. Janet Mullington and colleagues demonstrated greater sleep continuity, or fewer arousals, by getting people to take a hot bath before sleep — with the rapid temperature decline that often presages more continual sleep during the night.
4. Temperature manipulations, like exercising three to five hours before sleep especially in non-athletes, leads to body core temperature decreases that seem to improve overall sleep continuity.
5. Get more exercise. Fitter people sleep better through the night. Athletes not only sleep with fewer interruptions, but also appear able to fall asleep even after intense exercise, and to sleep through more noise than others.
Rest matters. “Good” sleep means continual sleep — uninterrupted even for brief periods. There are many effective ways to obtain it.
Yet the Internet may circumvent some of them. People want to be connected 24 hours a day. What they don’t realize is that frequent Internet use can do more than change their brain — it will also change their bodies.
That may not be the kind of experiment you want to do on yourself.