As the days shorten, the brain products more of the chemical compound melatonin - a hormone that makes us feel more tired.
While the body’s natural internal clock (also known as your circadian rhythms) influences its production, the amount of light that you’re exposed to each day also helps to determine how much melatonin is made.
The release of melatonin is tied to sunset, so when the sun sets earlier, we feel tired sooner than we would do in the summer.
For people living in the northern hemisphere, tiredness can be even more noticeable as it can get dark when many people are still in the middle of their working day.
Since most of us don’t go to bed the moment the sun sets, the effect of melatonin released eventually wears off. You might notice that you feel sleepy at sunset, but then actually feel much more awake only a couple of hours later. You may even find yourself staying up later than at other times of year - but that the moment your alarm goes off you feel sleepier than ever.
As the days get shorter we become more and more out of sync with our circadian rhythms, and tiredness becomes more pronounced.
But it’s not only your energy levels that suffer. The brain uses the chemical serotonin to produce melatonin, so as melatonin goes up, serotonin levels go down. Serotonin is a major contributor to feelings of well-being and happiness and so if serotonin gets too low, we may feel anxious or depressed and even have additional trouble sleeping.
So what can you do?
Well, the answer is to trick your brain into thinking that it is actually summer!
Open your blinds or curtains as soon as you get up and try to get outdoors in natural daylight as much as possible. Even a brief walk at lunchtime will help.
Stick to your sleep schedule
It's tempting to go hit the snooze button when winter hits, but try to resist. You don’t need more sleep in winter than at any other time of the year. Aim for eight hours a night and try to go to bed and get up at the same time every day.
Exercise may be the last thing you want to do when you're feeling tired on dark winter evenings - but you might be surprised by how energetic you feel after working out. Exercise in the late afternoon may help to reduce early-evening fatigue and also improve your sleep.
Eat like it’s summer
Being overweight or underweight can affect your energy levels and leave you feeling sleepy. So it's important to make sure you eat a healthy, balanced diet.
Once the summer ends, there's a temptation to give up salads and fill up on comfort food. However, you'll have more energy if you include plenty of fruit and vegetables in your meals.
You may also find that your sweet tooth goes into overdrive in the winter months, but try to avoid foods containing lots of sugar. They may give you a rush of energy, but it's one that wears off quickly.