Abstract Midwinter insomnia (M1) is an initial type insomnia that is typically seen north of the Polar Circle during the “dark period”, when the sun docs not rise above the horizon. The cause of MI is not known, but it seems reasonable to assume that it is (he expression of a phase delay of the sleep-wake cycle, due to lack of the entraining effect of normal daylight. Based on his hypothesis, we have studied the effect of intensive light exposure (2000-2500 lux for half an hour between 7.30 and 8.30 a.m. for 5 days) on selected sleep and endocrinological variables (the latter will be reported elsewhere) in nine subjects with typical MI and eight healthy controls. After light exposure, the MI subjects had a significantly shortened sleep latency and a nonsignificant increase in total sleep time. Before light exposure, the MI subjects reported significantly less drowsiness in the evening than in the morning, whereas the opposite was true after light exposure. No significant changes were seen in the control group. The results of this study give some support to the delayed phase hypothesis.