This study explored a special practice engaged in exclusively by Muslims and evaluated its influence on their sleep patterns. No study has examined the effect of sleep interruption due to the Fajr prayer on sleep architecture or daytime sleepiness. Our findings demonstrated that splitting sleep because of the Fajr prayer does not affect sleep architecture or increase daytime sleepiness when the total TIB and sleep duration are maintained.
There was no difference between the two sleep patterns in terms of subjective or objective daytime sleepiness. For objective assessment of daytime sleepiness, an MSLT was used. The MSLT is a sensitive test that has been used to assess daytime sleepiness and has also been used in previous studies to assess daytime alertness after various naps and split-sleep protocols.[16
] Previous studies that tested experimental split-sleep protocols reported no impact on daytime vigilance when the total sleep duration was fixed. In a protocol by Nicholson et al
. that compared consolidated (8 hours) and split sleep (two 4-hour sleep periods separated by 10 hours of nocturnal wakefulness) in six healthy volunteers, no difference in daytime performance between the two protocols was reported.[17
] In addition, two experimental studies reported an increase in subsequent daytime performance after the split-sleep protocol compared with the consolidated protocol when the total sleep duration was maintained.[16
] However, neither study controlled for the duration of prior wakefulness at the time of testing, which might have influenced the measured endpoints (daytime performance).[6
] To avoid this problem, we monitored the participants’ sleep patterns one week before the start of the study to assure the adequacy and regularity of prior sleep and to avoid the effects of prior sleep deprivation. Recent experimental data support the view that performance is a function of total daily TIB, independent of whether the sleep was consolidated or split.[19
] On the other hand, circadian phase has a profound effect on both the efficiency and structure of sleep.[20
] However, this effect was negligible in our study in as much as the sleep periods of the two protocols almost coincided, with a difference of only 45 minutes. Sleep inertia is another important factor that may influence the interpretation of experiments designed to measure the impact of split-sleep protocols on neurobehavioral performance.[21
] To avoid the potential effects of sleep inertia, the participants in this study took their first MSLT nap at the same time of the day, approximately two hours after waking up.
While SWS dominates at the beginning of the night (the first one third), REM sleep episodes are longest in the last third of the night. The preferential distribution of SWS toward the beginning of a sleep episode is not thought to be mediated by circadian processes, but shows a marked response to the length of prior wakefulness.[22
] The SWS pattern reflects the homeostatic sleep system (sleep debt), which is highest at sleep onset and diminishes across the night as the sleep pressure wanes. This explains the higher proportion of stage N3 in the first sleep period in the split-sleep protocol. On the other hand, rapid REM sleep follows a circadian pattern,[23
] in which it increases in the morning during the last third of sleep when the pressure for sleep is less.[23
] The effect of a split-sleep schedule on REM has not been properly assessed. As expected, the proportion of REM sleep was significantly higher in the second sleep period in the split-sleep protocol, and the REM sleep latency was shorter; however, the overall REM percentage and duration were not different between the consolidated and split-sleep protocols. Evidence has supported a difference between REM density and REM sleep.[11
] Although REM sleep has exhibited a circadian pattern, it has been hypothesized that REM density is an index of sleep need and sleep satiety and is much less susceptible to circadian modulation.[11
] There is a sleep-dependent modulation of REM density, with the greatest density occurring when the sleep pressure is low. We found no difference between the sleep protocols in terms of REM density in the last third of sleep time (last two hours of sleep). This finding suggests that the sleep pressure in the last third of sleep in both protocols was comparable.
The WASO was significantly lower in the second sleep period of the split-sleep protocol. This could be related to the timing of sleep. In experimental studies, nap timing has been shown to have a significant effect on sleep efficiency and WASO.[16
] Kubo et al
. assessed the effect of nap timing on sleep efficiency and found that the sleep efficiency is greater and lower WASO scores occur when naps are taken between 4:00 and 6:00 AM than when naps are taken earlier in the night.[26
There are limitations of this study that need to be addressed. First, some Muslims are used to wake up for Fajr
prayer and do not go back to sleep. Intentionally, this group was not investigated in this study, as they usually have shorter sleep duration, which may affect their daytime function and sleepiness. During summer in Riyadh city, night prayer (Esha
) ends around 9:00 PM and Fajr
prayer comes around 3:30 AM. Another limitation is the fact that the mean sleep latency during MSLT of the studied group was on the lower side (7-8.5 minutes). It is agreed that a mean sleep latency of <5 minutes during MSLT indicates a pathological degree of sleepiness. On the other hand, sleep latencies between 5 and 10 minutes imply moderate sleepiness with less well-defined pathology and consequences.[27
] It is known that there is variation is the range of mean sleep latency among populations, which may be related to individual differences in sleep tendency.[27
] To control for that effect, we used the same controls in a cross-over design. In addition, we ruled out disorders that may increase daytime sleepiness prior to enrollment and monitored sleep duration using actigraphy before commencing the study to eliminate the possible effect of prior sleep deprivation. The current study supported our hypothesis that splitting sleep for the Fajr
prayer does not affect daytime sleepiness when the total sleep duration was maintained. The sleep architecture of both protocols was similar. The findings of the current investigation indicated that further studies of the effects of the two schedules on neurobehavioral and cognitive function are merited.