April is allegedly the cruellest month, but October and November could give it a run for its money. In the northern hemisphere, the nights are noticably drawing in, especially now the clocks have gone back and sunset is an hour earlier. Every day, it gets darker two minutes earlier than it did the day before. And, while there are compensations – hello moon! Hello stars! Hello, most of all, Orion the Hunter, back in our northern skies after long months below the horizon – many of us cannot be consoled. As one of his patients told the US psychiatrist Norman Rosenthal: “I didn’t realise how quickly we haemorrhage light”.

“I love that expression ‘the haemorrhaging of light,’” Rosenthal says. “With a haemorrhage, you feel depleted, you feel exhausted. It dramatises something that’s real.” According to Rosenthal, who has spent more than 30 years studying the effects of seasonal changes on mood, about one in five Americans suffer some sort of “winter blues”. He is one of them. When we speak, it is 8.30am in his hometown of Bethesda, Maryland, and he is sitting in front of a light designed to reduce the symptoms.

Things are even worse in the UK, especially after the kind of long, hot summer that we have just enjoyed. “Cloud cover is very important,” Rosenthal explains. “Historically, Britain is a cloudy nation. It’s an island nation, there’s a lot of fog and cloud coming off the sea. More recently, your summers have been more like ours – hot and sunny and dry. Now, when you have a hot, sunny and dry summer and a cloudy, dark winter, that’s when the seasonality of the problem becomes more apparent.” The further north you go, the gloomier it gets. In the winter, there are parts of northern Scotland that get an average of just 64 minutes of sunlight a day, according to the Met Office.

Perhaps a quarter of those with winter blues develop the extreme form known as seasonal affective disorder, or Sad. “This is characterised by the typical features of depression – poor sleep, low mood, irritability, social withdrawal and perhaps overeating, too,” says Sally Norton, a consultant surgeon and weight-loss expert with an interest in Sad. “It is probably, in part, related to hormonal changes around those chemical messengers such as serotonin and melatonin involved in regulating our body clock.” Mark Winwood, a director of psychological services for Axa PPP healthcare, adds that people with Sad “may find it very difficult to wake up in the morning and can often feel sleepy during the day. You may crave chocolate and high-carbohydrate foods, such as white bread or sugary foods. And you probably won’t be doing as much physically, so it’s easy to put on weight.”

Sad and the winter blues are sometimes described as separate conditions, but Rosenthal prefers to talk of a “spectrum”. “Some people are really laid low by it and some people are just hampered by it. Let’s say your wife has Sad and you’ve got a little bit of winter blues. You get up in the morning and you don’t feel as good as you do in the summer, but you get yourself together and by the middle of the morning you’re not feeling so bad. But your wife can’t get out of bed at all. And you say: ‘I don’t know what’s the matter with you, dear. I get up, I go to work …’ It’s important to realise that a person isn’t fixed in a category. They can move to and fro depending on what they do and how bad the winter is.”

So, what can you do about it? “‘Take it seriously’ is always my first piece of advice,” says Rosenthal. “You Brits are famous for your stiff upper lip and just getting on with the job, but that is the philosophy of minimising and denial. If there was nothing you can do about it, fine – but there is.”

The worst cases may call for antidepressants or cognitive behavioural therapy. Otherwise, you could try spending a few hours every day in front of a lightbox, like the one Rosenthal uses. He recommends the specialist suppliers Lumie and the Sad Lightbox Company, and a box that will put out at least 10,000 lux. (By way of comparison, a sunny summer’s day might deliver 100,000 lux.) If you don’t spend much time at home, there are portable versions that are barely bigger than an iPad. “Morning treatments are best,” Rosenthal says. “The earlier the better.” He is also a fan of dawn simulators, which gradually light up your bedroom in the morning. Although winter’s late sunrises feel less oppressive than its early sunsets, that does not mean they don’t affect us.

Most of the advice for dealing with Sad or the winter blues boils down to: “Get more light.” You can start by making at least one room in your home as bright as possible, by adding artificial light, opening curtains, rolling up blinds, cleaning your windows and trimming any plants that obscure them. Then, even if it is raining, snowing or blowing a gale, get outdoors as much as possible. As Winwood says: “Natural daylight supports our circadian clock and helps us sleep at night. So, even if you seem to go to work when it’s dark and also go home when it’s dark, get out in the lunch hour. Even better, use it as a time to do your daily exercise.” Exercise, after all, is known to improve your mood, whatever the time of year.

Even – especially – if you don’t think of yourself as a winter person, it is worth investing in warm clothes so the weather can’t keep you cooped up. While you are out, the sunlight will encourage your body to produce vitamin D. Although Norton says there is only “limited evidence” for vitamin D improving mood, “there is no doubt that many of us are deficient in the winter months”.

When the night falls, don’t fight it too hard. “Our tendency to turn on blazing lights as soon as the light drops is bound to confuse our body,” Norton says. “Try to keep lights dim in the evening and let your body clock tell you when it’s time to sleep. Getting to bed early enough to get a good eight hours will help your mood and so much more.” And stay away from screens for at least two hours before you go to bed. “They suppress the sleep hormone melatonin, making it more difficult to drift off and reducing sleep quality.”

You should also watch what you eat, she warns. “While low mood and poor sleep has many people reaching for high-carb, sugary food, this is unlikely to help in the long term, with weight gain and yo-yoing blood sugar making sleep more difficult and worsening mood further.”

The important thing, Rosenthal repeats, is to realise that you are not powerless. “I find people who say: ‘I can’t deal with it!’ And I think to myself: ‘Let’s just put everything in place for you to have the best possible winter.’

“It’s not just about managing winter,” he says. “I think you need to enjoy it. Life should be embraced.”

Norman Rosenthal’s Winter Blues is published by Guilford Press.