Cloudy days, a tired mind: does a lack of sunlight actually slow us down?

It’s grey outside, and it gets dark at four. Why on earth does it take me three attempts to remember the name of the colle­ague I’ve been bumping into in the coffee kitchen for the past three years? Is it down to me – or is it October?

There’s now a new argu­ment in favour of the ‘autumn excuse’. Rese­ar­chers at the Univer­sity of Edin­burgh have inves­ti­gated whether the amount of sunlight has a short-term effect on our mental perfor­mance. To do this, they linked data from two large British cohorts with the level of suns­hine on the day of a cogni­tive test and the day before. Results from tens of thou­sands of adults and young people were included in the analysis.

Words: yes; numbers: no

When there was more suns­hine on those two days, perfor­mance actually improved. In adults, this included, amongst other things, imme­diate and delayed recall of words, as well as verbal fluency. In adole­s­cents, corre­la­tions were observed in reading compre­hen­sion and maths. Ironi­cally, however, no effect was observed in adults when it came to number sequences. Appar­ently, at least this part of the brain is not affected by the weather.

The corre­la­tions were stron­gest when there was low levels of sunlight – as is typi­cally the case in winter and, to some extent, in autumn. At higher levels, the curve levelled off. So more suns­hine did not make the brain perform better indefinitely.

Light on for the brain?

And why does light help, then? It is possible that the circa­dian rhythm, mood and the release of nitric oxide – which also plays a part in regu­la­ting cere­bral blood flow – all play a role. Vitamin D, on the other hand, is not a very plau­sible expl­ana­tion for an effect occur­ring within a few days.

Please note: this is an obser­va­tional study and does not prove that a walk in the suns­hine imme­dia­tely gets the brain working at full speed. However, it does provide a deci­dedly health-promo­ting working hypo­thesis for the autumn: if nothing is working out today, perhaps you should just get out for a bit. And then maybe check that last set of figures you typed up in the office one more time.

Stevenson A & Dibben C. Acute lack of sunlight expo­sure is asso­ciated with poorer cogni­tive test perfor­mance: evidence from two national cohorts in the United Kingdom. Photo­chem Photo­biol Sci 2026.