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This study aimed at elucidating the physiological significance of dusk and dawn in the circadian rhythm of core temperature (Tcore) and urinary 6-hydroxymelatonin sulfate in humans during sleep and the waking sensation just after rising. Seven female and four male students served as participants. Participants retired at 2300 h and rose at 0700 h. They were requested to sit on a chair and spend time as quietly as possible during wakefulness, reading a book or listening to recorded light music. Two lighting conditions were provided for each participant: 1) Light-Dark (LD)-rectangular light change with abrupt decrease from 3,000 lx to100 lx at 1800 h, abrupt increase from 0 lx to 3,000 lx at 0700 h. 2) LD-twilight light change with gradual decrease from 3,000 lx to 100 lx starting at 1700 h (twilight period about 2 h), with gradual increase from 0 lx to 3,000 lx starting at 0500 h (twilight period about 2 h). The periods of 0 lx at night were from 2300 h to 0700 h on the first day and from 2300 to 0500 h on the second day. Nadir time advanced significantly under the influence of the LD-twilight condition. The amount of 6-hydroxymelatonin sulfate in urine collected at 0200 h was significantly higher under LD-twilight in comparison with LD-rectangular light. Morning drowsiness tended to be lower under LD-twilight. Our results suggest that in architectural design of indoor illumination it is important to provide LD-twilight in the evening and early morning for sleep promotion in healthy normal people and/or light treatment in elderly patients with advanced dementia.

The present experiment aimed at knowing how a gradual changes of room temperature (Ta) and light in the evening and early morning could influence circadian rhythms of core temperature (Tcore), skin temperatures, urinary 6-hydroxymelatonin sulfate and waking sensation just after rising in humans. Two kinds of room environment were provided for each participant: 1) Constant room temperature (Ta) of 27 °C over the 24 h and LD-rectangular light change with abrupt decreasing from 3,000 lx to100 lx at 1800,abrupt increasing from 0 lx to 3,000 lx at 0700. 2) Cyclic changes of Ta and with gradual decrease from 3,000 lx to 100 lx onset at 1700 (twilight period about 2 h), with gradual increasing from 0 lx to 3,000 lx onset at 0500 (about 2 h). Main results are summarized as follows: 1) Circadian rhythms of nadir in the core temperature (Tcore) significantly advanced earlier under the influence of gradual changes of Ta and light than no gradual changes of Ta and light. 2) Nocturnal fall of Tcore and morning rise of Tcore were greater and quicker, respectively, under the influence of gradual changes of Ta and light than no gradual changes of Ta and light. 3) Urinary 6-hydroxymelatonin sulfate during nocturnal sleep was significantly greater under the influence of gradual changes of Ta and light. 4) Waking sensation just after rising was significantly better under the influence of gradual changes of Ta and light. We discussed these findings in terms of circadian and thermoregulatory physiology.