Seasonal Affective Disorder: How the weather can cause 'hibernation'
The clocks have gone back, evenings are darker and temperatures have dropped, signalling that winter is on its way.
But while the excitement of Christmas, cosy nights in and the promise of brighter days ahead keep many people going, for some it is not enough.
Seasonal Affective Disorder, known as SAD, is a type of depression that comes and goes in a seasonal pattern.
While it can affect people at any seasonal change, it is most common in winter and impacts more women than men.
Caroline McMenamin, a mental health therapist from Londonderry, has suffered from SAD since she was a child.
She said it is a common misconception that SAD is simply winter blues and notes that Lana Del Rey sings about "summertime sadness" and TS Eliot writes about April being "the cruellest month".
"We all personalise how we feel, believing that it's 'just me', and so we don't speak up about it," the 30-year-old tells BBC News NI.
"This is why having a growing global conversation is so important because people will realise it's not just them, and that they're actually dealing with a very common condition."
According to the NHS, symptoms of SAD can include low mood, loss of pleasure, lethargy and craving carbohydrates.
It says it is often linked to reduced exposure to sunlight during the shorter autumn and winter months.
The main theory is that a lack of sunlight might stop a part of the brain - the hypothalamus - working properly, which may affect the body's internal clock as well as its production of melatonin and serotonin.
"It can affect appetite, sleep, hygiene, motivation, mood, sex drive, outlook on life, lack of energy and irritability," says Caroline.
Age NI says the condition "can make even those of us with the sunniest dispositions feel a little glum" and urged anyone suffering with their mental health to speak to a doctor.
'A palpable energy'
SAD strikes Caroline every year in September and fades around the beginning of spring.
"Until then, I have to be vigilant about my mental health," she says.
"As I've gotten older, I've come to manage it more as I've come to understand what it is, albeit I distinctly feel the symptoms come upon me when September comes around.
"There is a palpable energy and environmental change not to mention that September is associated with the beginning of school, and that has left a lasting impression on me, even though I've left school many years ago."
Lizzie Price, 65, noticed she had the symptoms of SAD when she was about 18, but said her feelings were "never given a title until about 10 years ago".
The retiree, who lives in Dromara, County Down, said: "I remember being about 20 and crying to my mum, saying 'everything in my life is going so good so why should I be feeling like this?'
"But it was the mid-70s and no one knew anything about it and didn't really talk about mental health issues."
Lizzie said her symptoms usually start around Halloween when "it is like a switch flicking", which happened last week.
"It's like real life hibernation. When I'm like this, I can sleep for 18 hours a day.
"I find it hard to concentrate so even reading, which I love, becomes difficult. I enjoy art too but, at my low points, I can't even do that."
'My mental health is more important'
Lizzie has found her own coping mechanisms but admits some days are a real struggle.
"I love going to the beach and getting fresh air, maybe a bit of cake as a treat, but sometimes it's hard," she says.
"I try to make a point of forcing myself out, when I can, to make myself feel good that day.
"My motto is 'put off to tomorrow what you can't do today' so I never force myself to do anything because I feel I ought to.
"If I need to tidy the house but I don't feel up to it, I don't do it. My mental health is more important."
If you or someone you know is struggling with issues raised by this story, find support through BBC Action Line.