Tired in winter? Here's the science behind seasonal fatigue

tired woman looking at her phone
(Image credit: Getty Images)

The long nights, short days and cold temperatures during winter can make  many people feel tired and sluggish, and perhaps even want to hibernate. But why does the season impact energy levels? 

"The arrival of winter brings with it a number of potential challenges for sleep, including reduced exposure to sunlight, cooler temperatures, clock changes, and lowered immunity," said Guy Meadows, a sleep researcher and co-founder of The Sleep School, an online platform offering science-based support around sleep. "All these [factors] disturb sleep, making it harder to get up in the morning."

Hormonal changes, unhealthy behaviors and certain vitamin deficiencies can also be behind the onset of seasonal fatigue.

For some people, however, the onset of winter can result in more than a feeling of blahness. For some, excessive tiredness, grogginess, lack of energy and low mood are symptoms of a  disorder known as seasonal affective disorder (SAD).

Below, we take a closer look at some of the most common causes of seasonal fatigue.

Seasonal affective disorder

SAD is a type of depression that occurs when the days get shorter in winter. According to the National Institute of Mental Health (NIMH), symptoms often start in the late fall or early winter and go away during the spring or summer.

Many symptoms of SAD are psychological, such as low mood, loss of interest in activities and feelings of hopelessness. There can also be physical symptoms, such as low energy, sleep problems or appetite changes, feeling sluggish and lethargic, having low energy, experiencing sleep problems and experiencing appetite or weight changes.

The NIMH notes that SAD is more common in women than men and tends to affect people who live further north, where winter days are shorter.

If you think you may have SAD, speak to your doctor, who will be able to devise an appropriate treatment plan. 


Hormones can affect energy levels. Serotonin, a hormone often tied to mood, and melatonin, a hormone that regulates sleep, maintain the body's circadian clock — its internal clock — which fluctuates with the seasonal night-day cycle. 

The brain's pineal gland is responsible for producing melatonin, which it does in response to darkness. However, because the winter months are often darker and have fewer hours of sunlight, this can alter melatonin production, making us produce more of the hormone in the morning (when it is still dark) and earlier in the evening. According to the Mayo Clinic, fluctuating levels of the hormone can play a role in developing SAD by altering sleep patterns and mood.

According to Meadows, being exposed to light immediately after waking up can help reduce the amount of daytime melatonin production. 

"When you expose yourself to natural or artificial light as soon as you wake up, the effects can be transformative," he said. "Having a regular ‘wind-up’ routine can help to entrain the brain to associate that time in the morning with being awake, helping to boost morning alertness and energy and reduce feelings of grogginess."

guy meadows
Guy Meadows

Guy Meadows is a sleep physiologist and clinical director of The Sleep School. He completed his doctorate in the philosophy of sleep disorders Imperial College London and has been working in sleep therapy since 2001. Meadows is pioneering the use of acceptance and commitment therapy for insomnia and runs therapist training workshops for the British Association of Behavioural and Cognitive Therapists (BABCP).  

Similarly, light therapy boxes are designed to deliver bright light as a non-invasive treatment for the symptoms of SAD, according to the Mayo Clinic. However, Mayo Clinic recommends talking to a health care provider before using a light box. 

Vitamin D

Vitamin D is essential for healthy bones and teeth and can help the immune system to resist bacteria and viruses, according to the Centers for Disease Control and Prevention (CDC).

Vitamin D deficiency can cause fatigue. In a 2016 study published in the journal Medicine, researchers administered a daily vitamin D supplement to a group of participants who had clinically low vitamin D levels, while another group of people who also had low vitamin D levels received a placebo. A rise in vitamin D levels correlated with significant improvements in self-reported fatigue.

According to the NIMH, vitamin D may also support healthy serotonin levels, which can drop during winter because of the rise in melatonin. This is because serotonin produces melatonin, so increased levels of melatonin depletes the serotonin supply. Low serotonin levels can result in a low mood and feelings of listlessness and sluggishness. 

However, taking vitamin D supplements without a diagnosed deficiency has not been demonstrated to have benefits, according to an editorial in the American Association of Family Physicians. Only a medical provider can diagnose a vitamin D deficiency.

Disruptions to the circadian rhythm

The circadian clock is very sensitive to light, so shorter days with a lack of exposure to sunlight, and longer evenings with more exposure to artificial light, can throw the circadian rhythm out of whack and cause sleep issues. 

Keeping the heat on in the evenings can also disrupt the circadian rhythm, according to the Sleep Foundation. The body’s core temperature naturally dips at night, and this dip helps people to sleep better. Warmer temperatures can disrupt this natural process. The Sleep Foundation recommends keeping the thermostat between 60 to 67 degrees Fahrenheit (16 to 19 degrees Celsius) for the optimal sleeping temperature. 

The American Academy of Sleep Medicine (AASM) also advises people to ensure the bedroom is not too hot or too cold to help get a good night’s sleep. This is because either extreme can cause the body to wake up during the night.

man looking tired sat working at his desk

(Image credit: Getty Images)

Diet changes during winter

Eating breakfast at the same time each morning can help to stabilize the circadian rhythm, according to Meadows. "It lets the body clock know the day has begun," he said. "This boosts alertness and energy levels and is an excellent start to the day."

Not exercising enough

Almost half of Americans delay physical exercise in the winter months, citing poor weather as an excuse to stay indoors, according to a 2019 study in the Journal of Sport and Health Science. Instead, they would hold off for days — or months — when the weather was a more comfortable temperature. 

However, this may mean missing out on valuable daylight and depleting overall energy levels. Physical activity is one of the best ways to stay energized throughout the day and can also improve mental health and sleep, according to the CDC. While it is possible to exercise indoors, able to exercise outdoors is an important part of maintaining a healthy amount of physical activity, according to a 2019 review in the journal Environmental Science & Technology.

Man running wearing a smartwatch. Westend61 via Getty Images

(Image credit: Westend61 via Getty Images)

Do we need more sleep during winter?

Around a third of Americans say they sleep more during winter, according to a survey by the AASM. But does that mean we actually need more sleep during the colder season?

Both the Sleep Foundation, a non-profit organization based in the U.S., and the AASM advise that adults should be getting around seven to nine hours of sleep each night. These guidelines don't vary with seasons, which suggests that humans don't require more sleep in the colder months. 

However, all the seasonal factors above can affect our sleep, Meadows said.

"Studies have shown that in countries with a distinct seasonal variation, people tend to go to bed and get up a little later in the winter than summer," he said. "The lack of natural daylight can explain this during the winter, which acts to delay our sleep/wake cycle."

This article is for informational purposes only and is not meant to offer medical advice. Talk to your doctor if you think you may have seasonal affective disorder, and before taking a vitamin D supplement.

Joanne Lewsley

Joanne Lewsley is a UK-based freelance writer and editor, covering health and lifestyle news and features. She mainly creates evidence-based health and parenting content and has worked with a number of global sites, including BabyCentre UK, Medical News Today, Fit & Well, Top Ten Reviews, and Yahoo!