"Is Six Hours of Sleep Enough?"
Conventional wisdom has it that over the last 50 years
sleep duration has declined in parallel with the increasing
prevalence of obesity, suggesting that an epidemic
of sleep loss is associated with the epidemic of weight gain.
Now, we have triple digit streaming TV channels, smartphones,
and tablets to keep us entertained well into the night.
"The hurry and excitement of modern life is quite correctly
held to be responsible for much of the insomnia,"
concluded one medical journal editorial.
But that was an editorial published in 1894.
Are we really sleeping that much less?
Since 1905, sleep duration in children and adolescents
has declined by a little over an hour a night.
However, child labor wasn't outlawed until 1938 though,
and so part of the explanation may be due to the exhaustion
of sweating it out in the mines, farms, and factories
in the early part of the last century.
Since 1970, youth sleep duration has only declined about
15 minutes per night,
and it's not clear if sleep duration in adults has changed much at all.
Based on 168 studies of objective measurements of sleep duration,
instead of just self-report,
sleep duration in adults -- objective total sleep time --
has't changed much since 1960.
Since 2003, average sleep duration in the United States
may have even gone up.
Now, of course, just because we don't have evidence
that there's been a growing epidemic of sleep deprivation
that doesn't necessarily mean we're getting enough sleep.
Maybe we weren't getting enough sleep 50 years ago either,
or since Edison's lightbulbs,
or since candles were invented 5,000 years ago.
How might we determine the optimal sleep duration?
One way would be to study millions of people and see
how many hours a night is associated with the longest lifespan.
Sleep is a great mystery.
A trait shared across animal species, sleep must be of vital importance
to survive natural selection pressures to eliminate such a vulnerable state.
Indeed, cringe-worthy experiments have shown that
keeping animals awake long enough is fatal within 11 to 32 days.
One of the functions of sleep that has been elucidated in recent years
is the clearance of toxic waste substances
that build up during the day
through a newly discovered drainage system in the brain.
This could help explain why those who routinely get
less than seven hours of sleep at night may be at increased risk
of developing cognitive disorders such as dementia.
Even a single all-nighter can cause a significant increase
in beta amyloid accumulation in critical brain areas,
a gummy substance implicated in the development of Alzheimer's disease.
The lowest risk for developing cognitive impairment was found
for those getting seven to eight hours of sleep a night,
which is the same optimal range found for diabetes risk,
based on 36 studies following more than a million people.
The increased risk associated with getting only six hours a night
compared to seven or eight is comparable
to the increase in diabetes risk linked to physical inactivity.
For death from all causes combined, there's been more than 50 studies
following hundreds of thousands of people for up to 34 years.
Sleeping too short and too long are both associated
with cutting one's life short,
with the apparent sweet spot at seven hours a night.
Seven hours may seem short, but that may actually be
what's natural for our species.
Scientists studied three isolated pre-industrial societies
across two continents and found a surprising uniformity.
Despite no electric lighting or gadgets,
they stayed up until about three hours
after sunset and then typically rose before dawn,
accumulating about a solid six and a half hours of sleep
out of about seven and a half hours in "bed."
A mechanism by which excess sleep might be harmful remains elusive,
and so the association between increased risk of death and disease
and sleeping nine or more hours a night has been largely dismissed
as implausible.
Maybe its reverse causation,
sickness leading to more time in bed instead of vice-versa,
or confounding factors
such as employment status.
After all, who gets to sleep in?
Those without a job to get to.
However, there is experimental evidence showing negative
health effects from insufficient sleep.
So in terms of sleeping
in the sweet spot, aim for at least seven hours of regular sleep a day.