The immune system in autumn: how light and sleep help your defences

Seven o’clock on the last Sunday in October. My mobile phone automatically set itself back an hour during the night, the clock on the oven is still on summer time, and it’s already getting light outside. For the next few weeks, it’ll be getting light earlier in the morning and dark as early as five in the evening. On Monday, someone on the train coughs into their sleeve; on Wednesday, a colleague cancels the running meet.

The viruses have been around all summer. The conditions under which your immune system operates have changed, and the clock change merely highlights what’s been happening for weeks: daylight is becoming scarce, evenings spent under artificial light are growing longer, and the heating is drying out the air. That’s exactly where you need to start, and it’s more effective than any effervescent tablet.

Your internal clock is located in the brain, just above the point where the optic nerves cross. It is reset every morning, and its most powerful timing signal is light, which is detected by specialised sensory cells in the retina. It is this clock that also sets the rhythm for your immune system. The number of immune cells in the blood fluctuates measurably throughout the day; during the day, the system is more on patrol, whilst at night it processes what it has experienced.

If mornings remain dark and evenings are spent in front of a screen, this signal becomes blurred. Yet even a grey autumn morning outdoors provides many times more light than reaches a well-lit office. Anyone who spends twenty or thirty minutes outdoors during the first half of the day in the weeks following the clock change gives their body clock a clear starting signal – and simply walking to work is enough to achieve this. In the evening, the opposite applies. With warm, subdued lighting and a mobile phone that is put away in a drawer earlier than usual, the sleep hormone melatonin rises at the right time.

Just how effective this is is demonstrated by an experiment carried out in San Francisco. Researchers monitored the sleep patterns of 164 healthy adults for a week and then introduced cold viruses into their noses. Those who had slept for less than six hours a night were 4.2 times more likely to fall ill than those who had slept for more than seven hours. Age, stress and smoking were of little significance in comparison.

During deep sleep, the immune system processes what it has been exposed to during the day and creates a memory of it, much like your brain does after a long day of studying. So if you start feeling tired earlier in the evenings in autumn, it’s not a dip in form. You’re perfectly fine to spend the extra hour you gain from the clock change in bed.

From October to March, the sun is so low in the sky in our part of the world that hardly any UV-B radiation reaches the skin, and vitamin D production practically comes to a standstill. Your body then relies on the reserves it built up during the summer, and how long these last varies greatly from person to person. If you want to know for sure, have your GP check your levels in late autumn before resorting to high-dose supplements. We’ve explained everything that vitamin D influences in the body in a separate article.

And in case you’re wondering whether a morning walk makes up for it: it affects your internal clock via your eyes, whereas vitamin D is produced in the skin. One does not replace the other.

Kurt Tropper, founder of hotelsINshape and a keen cyclist himself, feels the change in his own body every year: “In summer, I often wake up naturally at five or half past five and feel raring to go straight away. Now, by half past nine in the evening, I’m tired and notice that I’m not as productive as I used to be. I use this time to consciously take it a bit easier. My mountain bike rides are getting shorter, the road bike’s already in the garage, and when the weather’s nice, we go hiking more often again and enjoy the mountains.” He takes extra vitamin D during the darker months. “Here in the Chiemgau, there’s often higher fog than in my old home, so there are even fewer hours of sunshine.”

Any moderate exercise session flushes immune cells out of their reservoirs and into the bloodstream, from where they patrol the body. Studies show that people who are active most days of the week have significantly fewer days with respiratory infections than those who hardly exercise at all. The key is to keep at it. Light outdoor endurance exercise in autumn has the advantage of combining physical activity with daylight; read more about this in our article on Zone 2 training in autumn. When it’s pouring with rain outside, strength training at the gym or a yoga class will keep you in shape.

The sauna has its place too: long-term Finnish data shows that people who use the sauna several times a week are less likely to develop pneumonia. There’s a surprisingly honest study from the Netherlands on cold showers. Just over 3,000 working people ended their shower with 30 to 90 seconds of cold water for a month. They fell ill just as often as the control group, but missed nearly 30 per cent fewer days at work. The cold doesn’t seem to stop the infection, but it does appear to alter how much it slows you down.

If you’ve caught it, saunas, ice baths and intense training sessions are off the cards. If you have a fever, aching limbs or symptoms below the neck, this also applies to light exercise; you can find out why in our article on training with an infection.

A large proportion of your immune cells are located in the gut wall, and the bacteria there feed on fibre from vegetables, pulses and whole grains. Pumpkin provides beta-carotene, which the body converts into vitamin A to maintain healthy mucous membranes; a handful of walnuts supplies zinc and omega-3 fatty acids; and rosehips contain more vitamin C than lemons. Add to that plenty of protein from fish, eggs, lentils or game, as antibodies are also proteins. You’ll find three recipes in Fit through Autumn.

Then there’s the air from the heating. Thoroughly airing out rooms, drinking plenty of fluids and maintaining a humidity level of 40 to 60 per cent keep your nose and throat moist enough to ward off pathogens. You can still keep drinking your ginger tea with lemon, though. Only stir in the honey once it has stopped steaming.

Ultimately, autumn is above all a question of light. How much of it you let into your day is a decision you make every morning at the front door.

Why do we get ill more often in autumn?
Less daylight throws our internal clock – and with it, the rhythm of our immune system – out of sync. Added to this is dry air from central heating, which dries out the mucous membranes, and spending many hours indoors with other people.

How does the autumn time change affect the body?
The body’s internal clock takes a few days to adjust. In the weeks that follow, it gets light earlier in the morning. If you make the most of this morning light outdoors, you send a clear signal to your day-night rhythm and find it easier to fall asleep in the evening.

How much sleep does the immune system need?
At least seven hours a night is a good guideline. In a study from San Francisco, people who slept for less than six hours were 4.2 times more likely to catch a cold than those who slept for more than seven hours.

When can I get my vitamin D from the sun in autumn?
Hardly at all between October and March. In our latitudes, the sun’s angle is too low; UV-B radiation is insufficient for the skin to produce vitamin D. The body then draws on its summer reserves. A blood test will reveal whether there is a deficiency.

Does vitamin C help against colds?
When taken regularly, vitamin C does not prevent colds in most people, but it can slightly shorten their duration. More important for the immune system are sufficient sleep, daylight, regular exercise and a varied diet.

Can I exercise whilst I have a cold?
If you have a mild cold without a fever, light exercise is usually fine. If you have a fever, aching limbs or symptoms below the neck, you should take a break from exercise, as otherwise the infection may drag on and, in the worst case, put strain on the heart.