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Background:  Disruptive effects of caffeine on sleep have previously been reported, although measures of next-day mood and performance have rarely been included. The present study aims to evaluate the effects of caffeine on sleep and associated next-day effects in a naturalistic field setting.  Methods:  Nineteen participants (daily caffeine intake 0-141 mg), assessed as good sleepers, took part in a randomized, placebo-controlled, double-blind, 2-week crossover study to assess the effects of bedtime caffeine use (250 mg) on sleep and next-day cognitive performance and mood, which were assessed on a mobile phone in the morning and afternoon. Sleep was assessed objectively (actiwatch) and subjectively (sleep diary).  Results:  Caffeine's effects on sleep were largely restricted to the first day of administration, with actigraphically measured reduced sleep efficiency, increased activity score and fragmentation index, decreased self-rated sleep quality, and an increased occurrence of participants waking early; only decreased sleep efficiency remained over the week. Effects on next-day performance and mood were evident over the whole week, although despite disrupting sleep, accuracy on a working memory task was higher after caffeine than placebo administration.  Conclusions:  Caffeine disrupted sleep, although when assessing next-day performance, which may have been affected by the presence of residual caffeine, performance appeared better after caffeine compared to placebo, although this was most likely due to prevention of the effects of overnight withdrawal from caffeine rather than representing a net benefit. Furthermore, partial tolerance developed to the effects of caffeine on sleep.

Naturalistic Effects of Five Days of Bedtime Caffeine Use on Sleep, Next-Day Cognitive Performance, and Mood