Nocturnal melatonin secretion is not suppressed by light exposure behind the knee in humans

Ocular light exposure can phase shift circadian rhythms and suppress nocturnal melatonin production. A recent finding suggests that extraocular light can also produce phase shifts in humans. We investigated whether extraocular light could also suppress melatonin secretion in humans. We assayed the salivary melatonin of 16 subjects during a baseline night and an experimental night in dim light (10-20 lux). The experimental night included either: (1) 3-h ocular light exposure (1000 lux, n = 6); (2) 3-h extraocular light exposure behind the knee (13,000 lux, n = 7) or (3) constant dim light exposure (10-20 lux, n = 3). Melatonin suppression occurred with ocular light but not with extraocular light or constant dim light.