We talk a lot about nutrients in winter. Vitamin D. Magnesium. Omega-3. But there is one essential input most of us are quietly deficient in, and it is not in a bottle... light.
As winter settles across the UK, daylight shrinks and many of us leave home in darkness and return the same way. Yet sunlight is not just a backdrop to our day. It is a biological signal that regulates immunity, mood, metabolism, hormones and sleep. When it fades, our physiology responds.
Let’s explore why sunshine is foundational for overall health, what happens when we don’t get enough...
Sunlight as a biological regulator
Human physiology evolved outdoors. Every cell in the body operates according to circadian rhythms, which are synchronised primarily by light entering the eyes and interacting with the brain’s master clock. Light exposure influences:
- Cortisol timing
- Melatonin production
- Immune signalling
- Neurotransmitter production
- Metabolic regulation
- Hormone balance
Disruption of natural light exposure patterns has been associated with sleep disturbances, mood disorders, metabolic dysfunction, and immune dysregulation. Light is information. And in winter, that information becomes limited (1,2).
The physiological benefits of sunlight
1. Vitamin D synthesis and immune regulation
Ultraviolet B (UVB) radiation enables the skin to synthesise vitamin D, which acts as a hormone within the body.
Vitamin D helps keep the immune system balanced and effective. It supports the body’s first line of defence against infections, regulates inflammatory signals, and helps prevent the immune system from becoming overactive. Because of this, adequate vitamin D levels are linked to stronger immune resilience and balanced immune function.
Low vitamin D status has been associated with increased susceptibility to respiratory infections and immune dysregulation (3,4).
In the UK, UVB rays are insufficient for meaningful vitamin D synthesis from approximately October to March, making deficiency common during winter months (5).
2. Immune function beyond vitamin D
Interestingly, sunlight influences immunity independently of vitamin D.
UV exposure can trigger nitric oxide release from the skin, influencing vascular function and blood pressure regulation (6). There is also emerging evidence that sunlight exposure may modulate T-regulatory cells and inflammatory pathways beyond vitamin D mechanisms (7).
This suggests sunlight has multi-layered immune effects, not solely mediated by vitamin D.
3. Mental health and neurotransmitter production
Sunlight stimulates serotonin production, a neurotransmitter central to mood regulation. Reduced light exposure during winter is strongly linked to Seasonal Affective Disorder (SAD) and depressive symptoms (8).
Bright light exposure has been shown to improve mood, enhance alertness, cognitive performances and regulate circadian rhythms. Light therapy is now an established intervention for seasonal mood disorders (9).
4. Sleep and hormonal synchronisation
Morning light exposure supports circadian rhythm, while daytime brightness strengthens the amount of melatonin release at night. Insufficient daytime light is associated with delayed sleep timing, poor sleep quality and reduces melatonin secretion (2).
5. Cardiovascular and metabolic effects
Based on studies, sunlight exposure has been associated with lower blood pressure, improved insulin sensitivity and reduced cardiovascular mortality (6,10).
While more research is needed, light exposure appears to influence metabolic regulation beyond vitamin D pathways.
What happens when we don’t get enough sunlight?
In northern countries like the UK, winter brings:
- Reduced vitamin D synthesis
- Increased rates of respiratory infections
- Higher prevalence of seasonal depression
- Circadian disruption
- Increased fatigue
Population data consistently show vitamin D insufficiency is common in the UK winter months, particularly among older adults, individuals with darker skin tones, and those who spend most time indoors (5).
Chronic insufficiency has been associated with increased autoimmune risk, impaired immune defence, reduced bone mineral density and mood disturbances.
Sunlight deprivation is not benign; it has measurable physiological consequences.
Sunlight across the lifespan
Children and adolescents: Vitamin D is essential for bone development and immune maturation. With increasing indoor lifestyles, deficiency risk is rising. Adequate light exposure also supports sleep regulation and mood stability.
Adults: Vitamin D plays a critical role in immune resilience and metabolic regulation, supporting stress hormone rhythms as well as mental performance and emotional balance.
Older adults: Reduced skin capacity to synthesise vitamin D increases deficiency risk, making older adults more susceptible to fractures and immune decline.
Each life stage depends on adequate light input, though capacity and exposure patterns differ (3).
6 Practical, cost-effective ways to optimise sunlight exposure in winter
In the UK, winter UVB is limited, but light exposure still matters for circadian and neurological health.
- Get morning light daily - Aim for 10–20 minutes outdoors within 30–60 minutes of waking, even on cloudy days. Outdoor light intensity far exceeds indoor lighting.
- Maximise midday daylight - Spend time outdoors between 11am–1pm when possible. While vitamin D synthesis may be minimal in winter, circadian and mood benefits remain significant.
- Support vitamin D - intelligently Given limited UVB, Public Health England recommends supplementation during autumn and winter months (5). Testing can help personalise dosage.
- Keep indoor environments bright - Work near windows. Take walking meetings. Step outside during breaks.
- Consider light therapy for mood - Clinically validated light boxes (10,000 lux) have demonstrated efficacy for seasonal affective symptoms (9).
- Combine light with movement - Outdoor walks amplify metabolic, immune, and mental health benefits.
The Takeaway
In winter, when exposure naturally declines, being intentional about light becomes even more important. While we cannot change the British weather, we can change our behaviour within it. Your immune system, sleep, mood, and metabolism are all listening to the light signals you provide. And sometimes, the most powerful interventions are also the simplest.
Step outside. Look up. Let biology do what it was designed to do.
