Ring the Bell for Matins: Circadian Adaptation to Split Sleep by Cloistered Monks and Nuns

Cloistered monks and nuns adhere to a 10-century-old strict schedule with a common zeitgeber of a night split by a 2- to 3-h-long Office (Matins). The authors evaluated how the circadian core body temperature rhythm and sleep adapt in cloistered monks and nuns in two monasteries. Five monks and five nuns following the split-sleep night schedule for 5 to 46 yrs without interruption and 10 controls underwent interviews, sleep scales, and physical examination and produced a week-long sleep diary and actigraphy, plus 48-h recordings of core body temperature. The circadian rhythm of temperature was described by partial Fourier time-series analysis (with 12- and 24-h harmonics). The temperature peak and trough values and clock times did not differ between groups. However, the temperature rhythm was biphasic in monks and nuns, with an early decrease at 19:39 +/- 4:30 h (median +/- 95% interval), plateau or rise of temperature at 22:35 +/- 00:23 h (while asleep) lasting 296 +/- 39 min, followed by a second decrease after the Matins Office, and a classical morning rise. Although they required alarm clocks to wake-up for Matins at midnight, the body temperature rise anticipated the nocturnal awakening by 85 +/- 15 min. Compared to the controls, the monks and nuns had an earlier sleep onset (20:05 +/- 00:59 h vs. 00:00 +/- 00:54 h, median +/- 95% confidence interval, p = .0001) and offset (06:27 +/- 0:22 h, vs. 07:37 +/- 0:33 h, p = .0001), as well as a shorter sleep time (6.5 +/- 0.6 vs. 7.6 +/- 0.7 h, p = .05). They reported difficulties with sleep latency, sleep duration, and daytime function, and more frequent hypnagogic hallucinations. In contrast to their daytime silence, they experienced conversations (and occasionally prayers) in dreams. The biphasic temperature profile in monks and nuns suggests the human clock adapts to and even anticipates nocturnal awakenings. It resembles the biphasic sleep and rhythm of healthy volunteers transferred to a short (10-h) photoperiod and provides a living glance into the sleep pattern of medieval time. (Author correspondence: isabelle.arnulf@psl.aphp.fr)