Ключевые слова: 24hour cycle, alertness, body clock, chronotype, circadian rhythm, contrast, darkness, dawn, day, daylight, diurnal rhythm, dusk, german, insomnia, jet lag, lightdark cycle, melatonin, morning lark, night, night owl, nocturnal rhythm, opposites, productivity, relaxation, rest, shift work, sleep deprivation, sleepwake cycle, sunrise, sunset,