Why You Feel Down Every Winter - Seasonal Affective Disorder and Light Therapy

Feeling Low in Winter Isn't "Just in Your Head"

As autumn deepens and daylight shrinks, a heaviness settles in without clear reason. You can't get up in the morning. You crave sweets uncontrollably. Everything feels like too much effort. When these symptoms recur at the same time every year, you may be experiencing Seasonal Affective Disorder (SAD).

SAD is not merely "the winter blues" but a biological phenomenon: shorter daylight hours affect neurotransmitters in the brain. Reported prevalence shifts considerably with how it is measured. In a population survey from Iceland, a high-latitude country, reported at the end of 2025, 2.24 percent of people met a SAD diagnosis on clinical interview, while 17.86 percent showed large seasonal swings in mood and behaviour. The long-held view that SAD grows more common the higher the latitude is now being re-examined in the light of findings like these. If you feel that every winter throws you off, understanding the mechanism matters more than blaming yourself.

Daylight Hours and Serotonin

The leading explanation for SAD is a change in how the brain's serotonin system works as daylight hours shrink. Serotonin is a neurotransmitter involved in stabilising mood, regulating appetite, and maintaining sleep rhythm. Its raw material is tryptophan taken in from food, but how actively the serotonin system runs is governed by the amount of light reaching your eyes. In winter, with fewer daylight hours, the efficiency with which serotonin is used between nerve cells is thought to drop.

At the same time, longer hours of darkness increase melatonin (the sleep hormone), making drowsiness and sluggishness easy to feel even during the day. On top of that, when the body clock (the circadian rhythm) drifts out of step with the daylight pattern, the body fails to switch into waking mode at your usual rising time, and morning fatigue intensifies.

The Typical Symptoms of SAD

SAD's symptoms overlap heavily with those of ordinary depression, but a few differences stand out. Ordinary depression more often brings loss of appetite and insomnia, whereas SAD is marked by overeating (a craving for carbohydrates in particular) and oversleeping. Weight gain, strong daytime sleepiness, and a heavy, leaden feeling in the limbs are also characteristic of SAD.

Symptoms usually begin in October or November and improve on their own between March and April. When this seasonal pattern is seen for two or more consecutive years, a diagnosis of SAD becomes considerably more likely. Many people let it pass on the grounds that spring will fix it, but a drop in quality of life lasting several months every year is not something to overlook.

Light Therapy - The Treatment With the Strongest Evidence

Light therapy is the treatment whose effect is best demonstrated for SAD. Bathing in high-intensity light of 10,000 lux for 20 to 30 minutes every morning promotes serotonin activity and resets the body clock. The effect usually begins to appear within one to two weeks.

The key to light therapy is doing it in the morning. Taking in light within an hour of waking suppresses melatonin and brings your waking rhythm back into line. Place the light box about 40 to 60 centimetres from your eyes; there is no need to stare into it. It works while you eat breakfast or read the paper.

When choosing a commercial light box, check that it delivers at least 10,000 lux, that it filters out ultraviolet light, and that the illuminated area is wide enough. Combining sleep quality improvement strategies with light therapy produces synergistic benefits.

The Role of Vitamin D and Diet

Fewer daylight hours also reduce the synthesis of vitamin D in the skin. Vitamin D is involved in serotonin synthesis, and research finds that the lower a person's blood vitamin D level, the higher their risk of SAD.

Topping up vitamin D through food or supplements is recommended over winter. Foods rich in vitamin D include oily fish such as salmon, mackerel, and sardines, along with egg yolk and mushrooms. If you use a supplement, 1,000 to 2,000 IU a day is the usual rough guide. Understanding the effects of vitamin D deficiency and appropriate countermeasures helps with overall winter health management.

Deliberately eating foods that contain tryptophan, serotonin's raw material (bananas, soy products, dairy, nuts), also contributes to steadier mood. Rather than surrendering to carbohydrate cravings with packet snacks, it matters that you choose carbohydrates that avoid a sharp blood sugar spike, such as whole grains and oatmeal.

Adjusting Exercise and Daily Rhythm

Aerobic exercise has the effect of easing SAD symptoms. Walking or jogging outdoors is especially useful because it delivers the benefit of exercise and exposure to natural light at once. Even on a cloudy day, outdoor light runs at 1,000 to 10,000 lux, far brighter than indoor lighting (300 to 500 lux), so a habit of stepping outside even briefly feeds into better symptoms.

Regularity in your daily rhythm matters too. Get up at the same time each day, take in light in the morning, move your body during the day, avoid screen light at night, and go to bed early. Stabilising this cycle corrects the drift in your body clock and settles the balance between serotonin and melatonin. Sleeping in at weekends shifts the body clock, so keeping your rising time within an hour of your weekday time is recommended.

Professional Consultation and Treatment Options

If light therapy and changes to daily habits do not produce enough improvement, consider seeing a psychiatrist or a psychosomatic medicine clinic. SSRIs (selective serotonin reuptake inhibitors) are sometimes used for SAD, and in some cases the option chosen is to combine them with light therapy.

Cognitive behavioural therapy (CBT) is applied to SAD as well. It corrects the negative thought patterns peculiar to winter ("I can't do anything in winter," "there is nothing to do but endure until spring") and works on behavioural activation by planning in activities that can be enjoyed even in winter. Because SAD tends to return every year, starting light therapy preventively around September, before symptoms set in, is another effective strategy.

Deliberately keeping up social activity through the winter also aids recovery. If you cut down on going out because of the cold and the dark, isolation deepens and symptoms worsen. Seeing a friend even once a week, joining a hobby group: holding on to social ties softens the dip in mood and becomes the support that carries you through winter. Turning winter from a season to be endured into a season to enjoy with a little ingenuity is what leads to long-term prevention.