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What happened to sleeping 8 hours?
People Live Longer Limiting Sleep to Seven Hours,
Study Says
Feb. 3, 2004 - The battle of the researchers over how much sleep
humans need continues with the lastest study saying people live longer
if they sleep seven hours, rather than the often recommended eight
hours. - or less than 4.5 hours.
An editorial, titled Do We Sleep Too Much? in the
February 2004 issue of the journal SLEEP by Daniel F. Kripke,
M.D., professor of psychiatry, University of California, San Diego (UCSD)
School of Medicine, comments on a study by A. Tamakoshi and Y. Ohno in
the same issue of the journal, as well as the results of two other
studies.
The Editorial from SLEEP
How Much Should An
Adult Sleep?
The JACC study from
Japan, reported in this issue of
SLEEP by Tamakoshi, Ohno, and colleagues, suggested:
1) The
best survival is experienced by those who sleep 6.5-7.5 hours on
weekdays.
2) The mortality risk of those who sleep more than 7.5 hours
is of more concern than the risk of those who sleep less than 6.5 hours.
Even those who
reported sleeping 8 hours had greater mortality risk than those who
slept an hour less. Although these conclusions might surprise
clinicians, the JACC data are fully consistent with recently-reported
results from the Nurses Health Study (NHS).2-4
and the Cancer Prevention Study II (CPSII).5
The consistency of
results among these three enormous studies, conducted with varying
methodologies on two continents and over two decades, suggests that they
are likely to prove reliable. A dozen smaller studies have supported the
general findings of the large studies. No persuasive epidemiologic
evidence contradicts them.
A strength of the
JACC study was that it specifically asked about average sleep duration
on weekdays, clarifying what was meant. The average sleep duration was
7.58 hours for men and 7.12 hours for women and increased somewhat with
age.
For men in JACC, the
lowest mortality was 1 hour less than the mode, as was the case for men
and women in CPSII. There were fewer nurses who slept 8 hours, possibly
due to nursing duties, so the NHS mode of 7 hours was also the point of
minimal mortality, as it was for women in JACC.
In JACC, those who
slept 8 hours had very significantly higher mortality than those who
slept 7 hours, regardless of the extent to which the data were adjusted
for comorbidities.
The same was true in NHS and CPSII. The
fully-adjusted mortality risk ratios for women who slept 8 hours (as
compared to 7 hours) were 1.30 (1.12-1.52 95% C.I.) in JACC, 1.13 in NHS,
and 1.13 (1.09-1.16 95% C.I.) in CPSII.
These estimates seem
in excellent agreement, considering that the age ranges were somewhat
different. Moreover, in JACC as in the other studies, those who slept 9
hours or 10 hours and more had progressively increasing mortality risks.
The increases in mortality of those who slept 8 hours (7.5-8.5 hours)
compared to 7 hours (6.5-7.5 hours) were quite small, but because of the
large numbers, would represent a substantial proportion of total
population mortality.
The results falsify the
widely-circulated hypothesis that it is best to sleep at least 8 hours.
In the JACC models
most fully adjusted for comorbidities, those who reported 5 or 6 hours
sleep per week night did not have significantly increased mortality
compared to those who slept 7 hours. Only those who reported less than
4.5 hours sleep (i.e., 4 hours or less) experienced significantly
increased mortality. Data of the NHS were similar.
In the very large
CPSII sample, the risk ratios of those who slept 5 and 6 hours were only
slightly elevated compared to 7 hours, though statistically significant.
The three studies were therefore consistent in showing more mortality
risk above 7.5 hours sleep than below 6.5 hours sleep, especially since
a higher proportion of the subjects reported sleeping 7.5 hours or more
as compared to less than 6.5 hours.
In conclusion, there
is more excess mortality associated with sleep above 7.5 hours than
below 6.5 hours. Long sleep was associated epidemiologically with more
of the populations excess mortality risk than short sleep.
A strength of the
JACC study was an attempt to control for mental stress and depression.
Although the questions used to indicate depression did not correspond
well with validated depression scales, the JACC scale was to some extent
validated by its usefulness for predicting mortality.
In JACC, 2 or more
depressive symptoms were observed least among those reporting 7 hours
sleep, with depressive symptoms occurring more among both those
reporting both shorter or longer sleep. A similar U-shaped relationship
of depression to hours of sleep was observed in the Womens Health
Initiative.6
In contrast, in a linear fashion, the more subjects slept, the less they
reported mental stress.
If the underlying
cause of mortality associated with sleep durations were mental stress or
depression, one would expect that control for mental stress and
depression would reduce the risk ratios associated with sleep duration.
To the contrary, control for mental stress and depression tended to
increase the JACC risk ratios associated with short and long sleep,
suggesting that reported sleep duration was not a proxy for emotional
symptoms.
Similarly, in NHS,
adjustment for depression had no substantial effect on risk ratios
associated with sleep longer than 7 hours.4
In the JACC study, BMI (body mass index) had virtually no relationship
to reported sleep duration. There was little relationship of body mass
index to sleep duration in NHS also, though a more complex relationship
was observed in CPSII subjects. Since body mass index is an extremely
important correlate of sleep apnea (along with gender and age, all of
which were controlled), it seems unlikely that sleep apnea could explain
associations of reported sleep duration and mortality.
In NHS, the least
regular snoring was observed among the 7 hour sleepers, but adjustment
for snoring did not appreciably change the mortality risks of sleeping
more than 7 hours.2,4
Another strength of the JACC study was an analysis excluding events
which occurred within 2 years after entry. This tended to exclude
subjects who were moribund at entry, and whose abnormal sleep duration
might be attributed to a terminal illness. Because the risk ratios were
not materially altered by this exclusion, it is unlikely that the
association of sleep durations and mortality can be explained by
terminal illnesses.
The JACC study also
found that those reporting sleep longer than 7 hours were less educated,
less likely to have a spouse, and more likely to have a history of
stroke, myocardial infarction, or cancer. By these indices, long
sleepers were evidently less well than those who slept 7 hours.
Nevertheless, control for these comorbidities did not substantially
alter the association of long reported sleep with increased mortality
risk.
Were there evidence
that those who sleep 8 hours or more had greater wellness, an argument
favoring 8 hours sleep might be offered, but JACC found several forms of
excess morbidity among those who slept 8 hours or more.
Too much credence
has been given to results implying that sleep restriction disturbs
glucose regulation, obtained from 11 subjects studied for a couple of
weeks without control for viral innoculation effects or experimental
order.7,8
The NHS evidence of 70,026 subjects studied for 10 years was more
reassuring in the most extensive multivariate model. NHS found no
significant relative risk of incident among those sleeping 5, 6, 7, or 8
hours, but a significant increase in relative risk among those sleeping
9 hours or more.3
In view of the mortality and morbidity findings of JACC,
clinicians should not recommend that adults sleep 8 hours or more.
The causal mechanisms of mortality associated with reported sleep of 8
hours and more have not been explained. Large epidemiologic studies have
all relied on self-reports of sleep duration.
Thus, we are not certain
to what extent those who report 8, 9, or 10 hours sleep actually do
sleep more. Perhaps respondents tended to report time-in-bed rather than
time asleep. Since prolonged time-in-bed is associated with increasing
sleep latencies and increasing wake-after-sleep-onset, those who report
8, 9, or 10 hours of sleep might actually experience little increase in
physiological sleep.
When presented with evidence that there is
increased mortality risk associated with 8, 9, or 10 hours sleep, many
with long sleep wish to know if they should voluntarily restrict their
sleep. This is an important unanswered question. Eventually, clinical
trials of sleep restriction or other interventions focusing on long
sleep will be needed, both to test the causal mechanisms for excess
mortality and to find how to reverse them. Explaining the risks
associated with long sleep will be a high priority.
News Report on this study - Click Here
Cancer
Prevention Study - Click Here
EditorialKripke
SLEEP, Vol. 27, No. 1, 2004
13 Do We Sleep Too
Much? Comment on Tamakoshi A;
Ohno Y. Self-reported sleep duration as a predictor of all-cause
mortality: results from the JACC study, Japan.
SLEEP 2004;27(1):51-4.
Daniel F. Kripke, M.D.
Professor of Psychiatry, University of
California, San Diego Address correspondence to: Daniel F.
Kripke, M.D., Professor of Psychiatry, University of California, San
Diego, 9500 Gilman Drive, La Jolla, CA 92093- 0667, ph: 858-534-7131,
fax: 858-534-7405, email: DKripke@UCSD.edu diabetes
REFERENCES
1. Tamakoshi A; Ohno Y. Self-reported
sleep duration as a predictor of all-cause mortality: results from the
JACC study, Japan. SLEEP
2004;27(1):51-4.
2. Ayas NT; White
DP; Manson JE; Stampfer MJ; Speizer FE; Malhotra A; Hu FB. A prospective
study of sleep duration and coronary heart disease in women. Arch Intern
Med. 2003;163:205-209.
3. Ayas NT; White
DP; Al-Delaimy WK; Manson JE; Stampfer MJ; Speizer FE; Patel S; Hu FB. A
prospective study of self-reported sleep duration and incident diabetes
in women. Diab Care. 2003;26:380-384.
4. Patel SR; Ayas
NT; White DP; Speizer FE; Stampfter MJ; Hu FB A prospective study of
sleep duration and mortality risk in women. Sleep 26 [Abstract
Supplement], A184. 2003.
5. Kripke DF;
Garfinkel L; Wingard DL; Klauber MR; Marler MR. Mortality associated
with sleep duration and insomnia. Arch Gen Psychiatry. 2002;59:131-136.
6. Kripke DF;
Brunner R; Freeman R; Hendrix S; Jackson RD; Masaki K; Carter RA. Sleep
complaints of postmenopausal women. Clinical Journal of Womens Health.
2001;1:244-252.
7. Spiegel K;
Leproult R; Van Cauter E. Impact of sleep debt on metabolic and
endocrine function. Lancet. 1999;354:1435-1439.
8. Spiegel K;
Sheridan JF; Van Cauter E. Effect of sleep deprivation on response to
immunization. JAMA. 2002;288:1471-1472. |
