Sleep patterns of European expatriates in a dry tropical climate

Night sleep in sedentary African subjects living in the sahelian zone lasts from 7 h to 8 h, with high amounts of slow-wave sleep (SWS) and paradoxical sleep (PS), SWS being present in each sleep cycle. We report here on sleep patterns in 6 healthy male European expatriates (aged 32-39 years) living in the same tropical climate. Polysomnography was taken for 3 consecutive nights in February (mean ambient temperature, Ta: 29.5 degrees C), March (Ta: 31.6 degrees C) and May (Ta: 33.3 degrees C). Comparisons between seasons were made with an analysis of variance, with P >/= 0.05. Because of a first night effect, the first nocturnal recording was discarded. Total sleep time (TST) increased in May vs February and March (P < 0.05). Stage 2 was shorter in March than in February (P < 0.001) and its proportion decreased from February to March (P < 0.02) and from March to May (P < 0.05). Conversely, SWS increased from February to March and March to May (duration, P < 0.001; proportion, P < 0.05), due to an augmentation in stage 4 with more numerous and longer stage 4 phases. Stage 3 was also increased in May vs March. The latency to SWS was shorter in March. SWS was present in each sleep cycle. PS was high, but did not vary. The sleep pattern changes were directly correlated with Ta. In conclusion, Caucasians living in the tropics slept similarly to Africans. The seasonal sleep variations favour the hypothesis that SWS is increased when thermoregulatory processes are triggered, either through passive climatic heating or exercise-induced hyperthermia.