The pattern of entrainment of the human sleep-wake rhythm by the natural photoperiod in the north.

Recently, it was shown that the sleep-wake rhythm of the inhabitants of the temperate zone is entrained to sun time. In the North, significant seasonal changes in the photoperiod may interfere with entrainment of the circadian system to sunlight. This investigation assessed the influence of photoperiod characteristics on the sleep length and sleep-wake rhythm of residents of high latitude. The study was conducted in four towns and six villages located between 59.5?N and 67.6?N latitude between the months of October and May from 2009 through 2011 and included 2822 subjects aged 10 to 97 yrs, 1621 of whom were females and 1201 males. The chronotype and sleep length of the subjects were assessed using the Munich Chronotype Questionnaire. The instructions for the questionnaire stressed the need to specify the sleep-wake schedule during the week preceding the date of completing the questionnaire. The study found that the length of sleep and the chronotype of the inhabitants surveyed in Northern European Russia depend on age, sex, type of settlement, and place of residence. The time of sunrise was a stronger predictor of sleep length and chronotype than the time of sunset and day length. A later chronotype and shorter sleep length were found for the subjects during the equinox (sunrise at 06:00 h) than under long-photoperiod conditions (sunrise at 04:00-05:00 h). During short-photoperiod conditions (sunrise at 07:00-10:00 h), no significant changes in the self-reported sleep-wake rhythm were found. The time of sunrise had the strongest impact on the sleep-wake rhythm of 30- to 97-yr-old persons. Sunrise had a stronger influence on chronotype and sleep length in January to May, when the days become longer, than in October to December, when the days become shorter. Age- and season-associated changes were found in the entrainment of the sleep-wake rhythm by photoperiod in the North.