The sun is at its furthest south over the Tropic of Capricorn at 4.19am tomorrow, making the northern hemisphere’s winter solstice unusually late this year. For those with seasonal affective disorder, a depression usually caused by lack of sunlight, it is a milestone on the way to better times, but the length of the day affects us all. When exposed to the sun we produce less of a hormone called melatonin and so feel more awake and energetic.

After dark the pea-sized gland in the brain that produces the hormone wakes up and releases melatonin into the blood making us feel tired and sleepy. This melatonin release lasts for about 12 hours, which accounts for why some people have difficulty getting up in the mornings, particularly at this time of year.

In America melatonin is widely taken as an over-the-counter medicine to induce sleep, but in the UK the NHS is more cautious and will only prescribe it to over 55s with certain sleep disorders.

It is the affect of light on melatonin production that leads to warnings about avoiding using computers and phones with bright screens in the hour before bed. Our brains are still working to the rhythm of sunlight and darkness and have yet to adjust to the invention of electric light.