Does too little sunlight give us all the winter blues?
The clocks have gone back, it is noticeably darker in the afternoons and sunlight is in short supply.
Are you feeling less energetic, craving more carbohydrates and chocolate and sleeping longer in bed?
Then perhaps the encroaching darkness, as autumn gives way to winter, is affecting your natural body rhythms. While many animals are preparing to hibernate, who would blame people for feeling ready to go into sleep mode too?
This is how nearly one in five of the UK population feel every winter between September and April, and particularly during December, January and February. They suffer from the 'winter blues', brought on by a reduction in daylight hours and a lack of sunlight.
Dr Aarohee Desai-Gupta, a psychiatrist in Enfield from The Royal College of Psychiatrists, says sunlight is key to our mood.
"The longer the period of sunlight, the longer the feeling of well-being generally. We have more energy, feel more active, more creative and happier."
So in winter it is unsurprising that the opposite happens.
Research from the University of Southampton found that most adults, at least 90%, experience subtle changes in moods, energy and sleep when the seasons change.
And an estimated one million working hours are lost by British industry over the winter months due to the winter blues.
If you recognise those feelings of lack of motivation and energy as the nights draw in then spare a thought for people who become seriously depressed by darkness.
Seasonal affective disorder or SAD, which affects 7% of people in the UK, is a full-blown depression.
It can be debilitating, it can prevent those affected from functioning normally and result in withdrawal from family and friends.
The cause of it all is darkness, says Dr Natasha Bijlani, consultant psychiatrist at the Priory Hospital in Roehampton.
"Winters with less daylight have more cases of SAD, except if there is snow on the ground. Snow reflects what light there is and makes the darkness more bearable."
SAD does not occur in countries around the equator. It is only a problem in parts of the world where the body's natural daily rhythm is disturbed by dawn and dusk coming closer together in winter.
But there is no need to start booking flights to the sun just yet: not everyone suffers from it.
Some people are thought to be more sensitive to light deprivation and more prone to stress, which can be a trigger.
A gland in the brain, which produces melatonin, controls how drowsy we feel. In darkness it makes us feel sleepy and in morning light it helps wake us up.
But in some people, this gland doesn't seem to work as well as it should, making them more vulnerable to SAD.
Helen Hanson has lived with seasonal affective disorder for 20 years. She first experienced a mystery winter illness aged 13 and then had no other problems until she was 40.
"I moved into a dark, terraced house and developed my first proper depression. It was a very physical experience. I had aches and pains and panic attacks and I realised there was an echo of the previous episode."
Over the next few years she experienced similar feelings around the month of September, before her GP spotted a pattern.
"I thought I was going mad. When I finally heard an expert talking about SAD, everything fitted. It felt wonderful to be able to give my feelings a name."
'Mushroom soup sky'
Helen says it is an illness that still surprises her because she can be affected at any time of year.
"If there's a mushroom soup sky I know I am in for trouble.
"But a good sunny week in February can be better than three solid weeks of rain August - and I don't necessarily feel great every July just because it's summer."
Helen copes with her disorder by using a light box "rigorously". She puts it on her bedside table alongside a daylight simulation lamp and sets them both to come on in the morning to help her gradually wake up and then feel like getting out of bed.
Light therapy has been shown to be effective in 85% of SAD cases, says Dr Bijlani.
"People with SAD need four hours a day of bright light at 10 times the intensity of ordinary lighting.
"It's a very simple treatment, but when used regularly throughout the winter months it can take away the worst of the feelings."
Helen knows she has to be on top of her condition all the time, particularly in November and December when it feels like she is going down into a dark pit.
"I've got to force myself to go outside and walk by the sea in the mornings.
"My body clock can go all wrong and I can feel exhausted and low, but I can't let up on it."