Winter Blues: Why Mood Can Drop in Winter - And How Light Can Lift It

Many people notice their mood dipping as winter arrives. Shorter days, colder weather, and less sunlight exposure can influence energy, sleep, and motivation. While “winter blues” is common, for some it becomes Seasonal Affective Disorder (SAD), a recognised subtype of depression linked to changes in light exposure. Understanding the biology behind this shift helps explain why mood can fluctuate throughout the year - and how simple light-based strategies can help.

The Circadian Rhythm Connection

Your circadian rhythm is a 24-hour internal clock that regulates sleep, alertness, hormones, appetite, and even emotional tone. Light is its strongest “time-giver”. When sunlight enters the eyes, it signals the brain’s suprachiasmatic nucleus (SCN) to regulate melatonin and cortisol, helping you feel awake during the day and sleepy at night (CDC, Effects of Light on Circadian Rhythms).

During winter, reduced daylight delays or blunts these signals. This can lead to:

• Increased melatonin production, making you feel groggy
  

• A flatter cortisol curve, lowering alertness and motivation
  

• Disruption of sleep timing, affecting mood regulation

Large-scale research confirms this link. A study of over 400,000 UK Biobank participants found that more time spent in outdoor light was associated with better mood, healthier sleep patterns, and improved circadian alignment (Burns et al., 2021).

The Role of Vitamin D

Vitamin D is produced when ultraviolet B (UVB) from sunlight interacts with the skin. In winter, UVB levels drop dramatically in northern latitudes - including the UK - making deficiency more likely.

According to the National Institutes of Health, low vitamin D is associated with fatigue, low mood, immune changes, and muscle weakness (NIH Office of Dietary Supplements). Supplementation is often recommended between October and March due to insufficient UVB for natural synthesis.

While sunlight is a natural source, expert bodies such as the American Academy of Dermatology emphasise using food and supplements for vitamin D rather than relying on deliberate UV exposure because of skin cancer risk.

Sunlight and Mood: More Than Vitamin D

Sunlight affects the body through several mechanisms beyond vitamin D:

a) Light exposure improves sleep and wellbeing

Office workers exposed to more daytime light report better sleep quality, mood, and overall health (Boubekri et al., 2014).

b) Sunlight may support cardiovascular health

Low-dose UVA can trigger nitric oxide release in the skin, relaxing blood vessels and reducing blood pressure (Liu et al., 2014; Weller et al., 2020). This may partly explain the “revived” feeling after morning light.

c) Light interacts with metabolic pathways

Emerging evidence shows fat cells contain light-sensitive receptors that influence metabolic regulation (Ondrusova et al., 2017).

d) Outdoor light counteracts rumination and low mood

Time outdoors is consistently linked with improved emotional wellbeing, likely due to combined biological and psychological effects.

Why We Don’t Only Feel Low in Winter

Although darkness exacerbates symptoms, low mood can occur year-round. Factors include:

• Chronic stress
  

• Poor sleep
  

• Indoor lifestyles
  

• Hormonal sensitivity
  

• Irregular schedules disrupting circadian cues

 Light exposure is a modifiable factor, and often overlooked.

Five Actionable Strategies to Support Mood

1. Get morning daylight (10–20 minutes)

Aim for within one hour of waking. Even cloudy UK light is strong enough to synchronise your circadian rhythm.

2. Take vitamin D daily in winter

Advice for adults and children over 4 years. It is recommended that everyone - including those who are pregnant or breastfeeding - takes a daily supplement of 10 micrograms Vitamin D (NHS, 2023). Vitamin D3 is usually the recommended supplement option, as it’s the same form your skin makes from sunlight and is more effective at boosting and maintaining healthy levels of 25-hydroxyvitamin D, the key marker used to measure vitamin D status.

3. Step outside during breaks

Natural light is up to 100 times brighter than indoor lighting. Short bursts across the day improve energy and regulate sleep.

4. Use Sunglasses Strategically 

Sunglasses are important for eye comfort and UV protection, but wearing them first thing in the morning can reduce the light signal your brain needs to switch off melatonin and fully wake up. Try to avoid wearing sunglasses in the morning so your eyes receive enough natural daylight to anchor your circadian rhythm - unless it is unsafe to drive without them, in which case safety always comes first.

5. Use a SAD lamp if needed

Look for 10,000 lux boxes used for 20-30 minutes in the morning (discuss suitability with a health professional if you have Bipolar Disorder or eye conditions).

A gentle reminder: Sunlight is beneficial, but excessive UV exposure increases skin cancer risk (American Academy of Dermatology; EPA; CDC). Prioritise safe light habits and avoid deliberate sunbathing.

References:

American Academy of Dermatology. (n.d.). Vitamin D: Stats and Facts. https://www.aad.org

Boubekri, M., et al. (2014). Impact of daylight exposure on health and sleep. J Clin Sleep Med, 10(6), 603–611.

Burns, A. C., et al. (2021). Outdoor light and mood/sleep outcomes. Journal of Affective Disorders, 295, 347–352.

Centres for Disease Control and Prevention. Effects of Light on Circadian Rhythms. https://www.cdc.gov

Hoel, D. G., & de Gruijl, F. R. (2018). Sun exposure public health directives. IJERPH, 15(12), 2794.

Liu, D., et al. (2014). UVA-induced vasodilation and blood pressure effects. J Invest Dermatol, 134(7), 1839–1846.

NHS. (2020, August 3). Vitamin D – Vitamins and minerals. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/

NIH Office of Dietary Supplements. Vitamin D Fact Sheet. https://ods.od.nih.gov

Ondrusova, K., et al. (2017). Light-sensitive pathways in adipocytes. Sci Rep, 7, 16332.

U.S. Environmental Protection Agency. Health Effects of UV Radiation. https://www.epa.gov

van der Rhee, H. J., et al. (2016). Sun exposure benefits. Med Hypotheses, 97, 34–37.

Weller, R. B., et al. (2020). Solar UV and blood pressure. JAHA, 9(5), e013837.