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Light is the strongest time cue for entrainment and phase resetting of the circadian clock.

In humans, exposure to long‐duration light (6.5 h) in the late evening/early night causes phase delays, suppresses melatonin and increases alertness.

Here we studied the effects of different durations of exposure to a single high‐intensity (∼10,000 lux) light pulse (0.2 h, 1 h, 2.5 h and 4.0 h) on phase shifting, suppression of melatonin and self‐reported sleepiness in young men and women.

Phase‐resetting and melatonin‐suppression responses were dose dependent and non‐linear; shorter light exposures more efficiently phase‐shift the clock, suppress melatonin and induce alertness.