Our brains are sharper in summer than in winter, a study has found.
It seems our minds are geared to be able to pay more attention when the days are long, with our ability to focus peaking at the time of the summer solstice in December.
But this is not just because our bodies react to longer days - our body clocks keep track of the seasons even when we cannot see daylight.
In a unique experiment, 28 volunteers were deprived of sleep for two days, then kept in dim light for a further three days.
They then performed computer tests of attention and memory while their brains were being scanned to see how they were functioning. The tests were done during various weeks over the course of a year.
The subjects performed better during the summer, suggesting their bodies were taking cues from an internal clock rather than external sunlight. Humans seem to be at their least attentive in the winter solstice, the study found.
Researchers from the University of Liege in Belgium said that for tasks that need sustained concentration, the best and worst responses "were located around summer and winter solstices respectively".
They also found that working memory - our ability to use and hold information for a short time - is at its best in the autumn equinox.