summarize: What is really striking about most of what we’ve discussed so far in this
chapter—the crucial role that sleep plays in metabolic health and
cardiovascular health—is how much of this effect is mediated through the
brain. Sleep plays a major role in brain health, especially as we get older, not
only in terms of daily cognitive function but also in terms of our long-term
cognitive health, a crucial pillar of healthspan.
We have all felt groggy and slow after a restless night; our brain simply is
not as sharp as it should be. A good night’s sleep or even a solid nap usually
restores us. But sleep researchers are revealing myriad ways in which good
sleep is essential to long-term brain health—and how bad sleep inflicts major
damage. Poor sleep was long considered to be one of the first symptoms of
incipient Alzheimer’s disease. Subsequent research, however, has pointed to
chronic bad sleep as a powerful potential cause of Alzheimer’s disease and
dementia. Sleep, it turns out, is as crucial to maintaining brain health as it is
to brain function.
When we get into bed and close our eyes, a series of physiological
changes begins to take place as we descend into sleep. Our heart rate slows,
our core temperature drops, and our breathing becomes regular as we wait for
sleep to overtake us. Meanwhile, our brain is embarking on its own journey.
Researchers now know that we sleep in a series of well-defined stages,
each of which has a specific function and a specific electrical brain wave
“signature,” which is how researchers initially identified the different sleep
stages. To visualize these stages, imagine that when you go to bed and close
your eyes, you are embarking on a deep-ocean dive in a submarine. As your
body relaxes, you fall asleep, hopefully within a few minutes, and your
metaphorical vessel slips beneath the waves and begins its descent.
Normally, our descent is quite rapid: we plunge into the depths, passing
through a period of light sleep before dropping into deep sleep. This sleep
stage is called non-REM, or NREM, sleep, and it comes in two strengths,
light NREM and deep NREM. The latter is the more important of the two,
especially for neurological health. In our submersible analogy, this is when we
descend into the lightless depths of the sea, where our brain is immune from
external stimuli. But that does not mean that there is nothing going on. As we
descend into deep sleep, our brain waves slow until they reach an extremely
low frequency, a chanting rhythm of about one to four cycles per second. This
deep sleep dominates the first half of the night, although we typically cycle
back and forth between deep and lighter NREM.
Later in the night, typically, our “submersible” rises back up toward the
surface, into a zone called rapid eye movement (REM) sleep. In this state, our
eyeballs really will dart around behind our eyelids. We are “seeing” things,
but only in our mind. This is where most of our dreaming occurs, as our mind
processes images and events that seem familiar but are also strange or
dislocated from their typical context. Interestingly, the electrical signature of
REM sleep is very similar to that when we are awake; the main difference is
that our body is paralyzed, which is probably not accidental, since it prevents
us from acting on our bizarre dream thoughts. It wouldn’t be good if we could
just get up and run around while we were in REM sleep. (This probably also
explains those dreams where we are trying to run away from something and
our body just won’t seem to cooperate.)
During a typical night, we will cycle between these sleep stages. These
sleep cycles last about ninety minutes, and we may even wake up momentarily
in between them—which is likely evolution’s way of protecting us from
getting eaten by a lion or attacked by enemies during the night, notes Dr.
Vikas Jain, a Stanford-trained sleep physician who works with me on my
patients’ sleep issues.
Both REM and deep NREM sleep (which we’ll call “deep sleep” for
convenience) are crucial to learning and memory, but in different ways. Deep
sleep is when the brain clears out its cache of short-term memories in the
hippocampus and selects the important ones for long-term storage in the
cortex, helping us to store and reinforce our most important memories of the
day. Researchers have observed a direct, linear relationship between how
much deep sleep we get in a given night and how well we will perform on a
memory test the next day.
When we are young, REM sleep is important in helping our brains grow
and develop. Even while we are asleep, our brain is forming new connections,
expanding our neural network; this is why younger people spend more time in
REM. In adulthood, our REM sleep time tends to plateau, but it remains
important, especially for creativity and problem solving. By generating
seemingly random associations between facts and memories, and by sorting
out the promising connections from the meaningless ones, the brain can often
come up with solutions to problems that stumped us the previous day.
Research has also found that REM sleep is especially helpful with what is
called procedural memory, learning new ways of moving the body, for
athletes and for musicians.
Another very important function of REM sleep is to help us process our
emotional memories, helping separate our emotions from the memory of the
negative (or positive) experience that triggered those emotions. This is why, if
we go to bed upset about something, it almost always seems better in the
morning. We remember the event but (eventually) forget the pain that
accompanied it. Without this break for emotional healing, we would live in a
state of constant anxiety, every memory triggering a renewed surge of the
emotions around that event. If this sounds like PTSD, you are correct: studies
of combat veterans found that they are less able to separate memories from
emotions, precisely due to their lack of REM sleep. It turned out that the
veterans put out high levels of noradrenaline, the fight-or-flight hormone that
effectively prevented their brains from relaxing enough to enter REM.
[*2]
Perhaps most intriguing, REM sleep helps us maintain our emotional
awareness. When we are deprived of REM, studies have found, we have a
more difficult time reading others’ facial expressions. REM-deprived study
subjects interpreted even friendly or neutral expressions as menacing. This is
not trivial: our ability to function as social animals
[*3] depends on our ability
to understand and navigate the feelings of others. In short, REM sleep seems
to protect our emotional equilibrium, while helping us process memories and
information.
Deep sleep, on the other hand, seems to be essential to the very health of
our brain as an organ. A few years ago, researchers in Rochester discovered
that while we are in deep sleep, the brain activates a kind of internal waste
disposal system that allows cerebrospinal fluid to flood in between the neurons
and sweep away intercellular junk; while this happens, the neurons themselves
pull back to allow this to happen, the way city residents are sometimes
required to move their cars to allow street sweepers to pass through. This
cleansing process flushes out detritus, including both amyloid-beta and tau,
the two proteins linked to neurodegeneration. But if we do not spend enough
time in deep sleep, the system cannot work as effectively, and amyloid and tau
build up among the neurons. Broader studies have found that people who
have generally slept less than seven hours per night, over decades, tend to
have much more amyloid-beta and tau built up in their brains than people
who sleep for seven hours or more per night. Tau, the protein that collects in
“tangles” inside unhealthy neurons, is itself correlated to sleep disturbances in
cognitively normal people and in those with MCI, or mild cognitive
impairment, an early stage of dementia.
This can become a vicious cycle. If someone has Alzheimer’s disease,
they are likely to experience sleep disturbances. People with Alzheimer’s
disease spend progressively less time in deep sleep and REM sleep, and they
may also experience dramatic changes in their circadian rhythm (i.e., sleepwake cycle). Also, up to half of people with Alzheimer’s disease develop
sleep apnea.
But sleep disturbances, in turn, may help create conditions that allow
Alzheimer’s to progress. Insomnia affects 30 to 50 percent of older adults,
and there is ample research showing that sleep disturbances often precede the
diagnosis of dementia by several years; they may even appear before cognitive
decline. One study linked poor sleep quality in cognitively normal people with
the onset of cognitive impairment—just one year later.
Meanwhile, superior sleep quality in older adults is associated with a
lower risk of developing MCI and Alzheimer’s disease, and with maintaining
a higher level of cognitive function. Successfully treating sleep disturbance
may delay the age of onset into MCI—by about eleven years, according to
one study—and may improve cognitive function in patients already diagnosed
with Alzheimer’s disease.
Clearly, sleep and cognitive health are deeply intertwined; this is why one
of the pillars of Alzheimer’s disease prevention, particularly for our high-risk
patients, is improving their sleep. It is not enough merely to spend time in
bed; good-quality sleep is essential to long-term brain health. This is the
crucial distinction. Sleep that is irregular, or fragmented, or not deep enough
will not allow the brain to enjoy any of these benefits.
Unfortunately, our ability to obtain deep sleep declines with age,
beginning as soon as our late twenties or early thirties, but worsening as we
enter middle age. It’s not entirely clear how much this decline in sleep quality
is due to growing older itself, versus the increased likelihood of health
conditions that result in poor sleep as we age. One analysis suggests that the
bulk of the changes in adult sleep patterns occur between the ages of nineteen
and sixty and only minimally decline after that, if one remains in good health
(a big if).
One possible contributor to this age-related reduction in deep sleep is
changes in growth hormone secretion. Growth hormone is typically released
in a pulse about one hour after we begin to sleep at night, or around the time
at which we’re likely to enter deep sleep. On the other hand, inhibiting growth
hormone reduces deep sleep, so it’s not clear which is cause or effect here.
Growth hormone reaches its peak during adolescence, rapidly declines
between young adulthood to middle age, and then declines more slowly after
that. This pattern parallels the changes in the amount of deep sleep we get as
we age.
More research points to the forties and sixties as the decades of life when
deep sleep is especially important for the prevention of Alzheimer’s disease.
People who have slept less during those decades seem to be at higher risk of
developing dementia later on. Thus, good sleep in middle age appears to be
especially important for maintaining cognitive health.
What I realize now is that for all those years when I was sleeping five or
six hours a night and thinking I was on top of my game, I was in fact likely
performing far below my potential, thanks to my lack of sleep. And at the
same time, I was probably putting myself at risk of long-term illness—
metabolic, cardiac, and cognitive. I always bragged, “I’ll sleep when I’m
dead.”
Little did I know that my not sleeping was doing much to hasten that day.