Why you feel more tired as the evenings get darker

By the end of September the streetlights are on before the washing-up is done. In London the sun sets at about ten to eight at the start of the month and at about twenty to seven by the end of it. Add the later sunrises and the day has lost nearly two hours in four weeks.

You feel it. Flagging at four in the afternoon, dozing on the sofa at nine, then oddly awake once you get to bed. The dark evenings get the blame for all of it. They are only part of the story, and they are the part you cannot change.

Your body clock runs on light

The clock in your brain that decides when you feel alert and when you feel sleepy sets itself mainly by light. Bright light tells it that it is daytime. Darkness lets it release melatonin, the hormone that makes you drowsy.

For that to work, day has to look very different from night. Outdoors it does. Full daylight measures around 25,000 lux, and direct sun can reach 100,000. A lit office is about 500, which is roughly what you get outdoors at sunset. A living room lit by lamps can be nearer 50.

In summer you get away with it. There is daylight on the walk to the station, daylight after work, daylight at eight in the evening while you put the bins out. In autumn the working day swallows nearly all of it. You leave in the half light, spend the bright hours at a desk, and come home in the dark.

What a dim day does

Your clock gets a weaker daytime signal to set itself by. At the same time, as the nights lengthen, the body releases melatonin for longer, and researchers have linked that to sleeping a little longer in the darker months. Wanting an earlier night in October is the season talking.

The NHS describes the same mechanism when it explains low mood in winter: reduced daylight may change the levels of melatonin and serotonin, which affect mood and sleep patterns.

Morning: get outside early

Daylight, and plenty of it, is what keeps your sleep from drifting later. In questionnaire research, each extra hour people spent outdoors went with falling asleep about half an hour earlier. Some of that may be the walking itself. You get both either way.

Morning is the easiest place to find that hour, because the evening is no longer on offer. Walk the first stretch of the commute instead of driving to the station. Get off the bus a stop early. If you work from home, go round the block before you open the laptop, and have the first cup of tea on the doorstep.

By late October the sun is not up in London until about twenty to eight, so an early alarm goes off in the dark. Get out once it is light, even if that means mid-morning.

Midday: the brightest hour is lunchtime

The sky is at its brightest around midday, which is exactly when a working day keeps you indoors. Take lunch outside, or at least to a seat by a window. The NHS self-care advice for people who struggle in winter is to get as much natural daylight as you can and to go outside in the daytime every day. Its advice on tiredness adds that even a single fifteen-minute walk can give you an energy boost, which is a better answer to the four o'clock slump than another coffee.

A grey day still counts. An ordinary overcast sky is about 1,000 lux, roughly twice the office you just walked out of.

Evening: keep it dim and stay off the sofa

The evening is when your body clock is most easily pushed later by light. After dinner, turn off the ceiling lights and use lamps. The NHS also advises against phones and tablets right before bed.

Two other habits creep in with the dark. The first is the nine o'clock doze in front of the television. Sleepiness builds the longer you are awake, and a nap on the sofa spends it before you get to bed. If you are nodding off, go to bed properly.

The second is the late coffee. The NHS says the effects of caffeine can last up to seven hours, so the cup that rescues your afternoon at four is still with you at eleven.

The clocks go back at the end of October

On the last Sunday of October the clocks go back an hour. In London the sunset moves overnight from about ten to six to about ten to five. The good news is on the other side of the day: mornings get an hour of light back. A habit of getting outside early, started now, carries straight across that weekend. If there is a teenager in the house, their clock already runs about two hours late, and the same morning light is what helps.

If it feels like more than tiredness

Feeling tired as the light goes is one thing. Low mood that settles in, or losing interest in things you usually enjoy, are among the symptoms the NHS lists for seasonal affective disorder, a type of depression that usually comes in winter. If that sounds familiar, the NHS advice is to see a GP.

You will see daylight lamps advertised at this time of year. The NHS says light boxes are not usually available on the NHS because there is not enough evidence that they work, though many people find they help, and it suggests speaking to a GP before trying one.

Tonight

After dinner, switch off the ceiling lights and leave the lamps on. Tomorrow, take your first cup of tea outside, whatever the sky is doing. Do both for two weeks, then see how four o'clock feels.