SLEEP
A large number of studies have demonstrated that
caffeine increases the time needed to go to sleep
and reduces the duration of sleep. However, there
are large individual differences in the effects of
caffeine on sleep. For example, in one study
caffeine given in the early morning had effects on
the subsequent nights sleep whereas in a second
study consumption of caffeine at bedtime had no
adverse effects on sleep the same night. One
explanation for these individual differences is that
habitual consumers of caffeine are less likely to
report sleep disturbances than infrequent
consumers of caffeine. Another explanation may
be that tolerance to the effects of caffeine on sleep
develops in habitual consumers and there are no
withdrawal effects when caffeine consumption is
discontinued. Even when caffeine consumption
produces sleep disturbances this may not be
translated into changes in mood and performance
the following day.
There is little evidence for associations between
caffeine consumption and sleep. A survey of 760
nurses showed that age and family factors were
more important determinants of sleep than caffeine
consumption. A second study could find no
association between caffeine consumption and
sleep in 72 shift workers. A study of sleep in elderly
women could find no differences in levels of
caffeine consumption between good and poor sleepers.
A 2002 review came to three conclusions about effects of caffeine on sleep. “First, large amounts of caffeine (e.g. over 3 mg/kg body weight in a single beverage) consumed in the late evening will prevent individuals from going to sleep and reduce sleep duration. Effects of smaller doses show large individual variation, with high consumers being more resistant to effects of caffeine on sleep. Secondly, the impact of caffeine-induced changes in sleep on behaviour the next day and long-term health is not known. Finally, high levels of caffeine consumption do not appear to be strongly related to sleep parameters.”
