When Spring Brings Frequent Colds and Shallow Sleep
The short answer
The daily clock the body set for winter has to be moved in spring. The suprachiasmatic nucleus, which does that work, shifts only a little each day. Meanwhile pollen thins the defence of the nasal lining, and a wide day-to-night temperature swing keeps the sympathetic nervous system switched on. Three things overlap, each in a different place.
Contents
Every spring I hear people say they seem to catch one cold after another. Sleep turns shallow, they say, and the day feels heavy for no clear reason.
Winter is over, so this ought to be the easier season. Why is it not? In spring, several things the body has to do arrive at once.
As the days lengthen, the body shifts its hours
The body keeps a clock that measures the day. Light entering the eye travels through the retina to the brain, and the place that receives that light sets the clock. It is a site that takes light information on its own, and it is called the suprachiasmatic nucleus.
In winter the sun rises late and sets early, and the body fixes its hours for falling asleep and waking to that light. Then spring comes and sunrise moves earlier week by week. The clock has to be reset.
That adjustment does not finish in a day or two. The body's clock moves only a little each day. So there are days when the world outside has already moved toward summer while the body is still keeping winter hours. That gap is why sleep comes hard in spring and mornings feel heavy.
Meanwhile the nose reacts first
Spring carries pollen. When pollen meets the lining of the nose, cells within that lining release histamine. These are called mast cells. Blood vessels open, the lining swells, the nose runs. Sneezing and itching come from here as well.
But it does not end at a blocked nose. The nasal lining is the first filter for everything we breathe in. When that lining swells and becomes sore, it filters less well. This is part of why allergy and colds so often arrive together in spring.
And a blocked nose means breathing through the mouth at night. Sleep turns shallow. This is where it meets the clock.
The body stays switched on
Spring has a wide swing of temperature within a single day. There are stretches of days when morning and afternoon differ by more than 10 degrees. Each time, the body narrows and widens its blood vessels to match.
A rapid change in the environment switches on the sympathetic nervous system. Itching and sneezing are rousing stimuli in their own right. A partly switched-on state carries through the day and the night.
Three things, each wearing away at a different place
It is hard to name one reason why people fall ill so readily in spring.
The nasal lining thins on the blocking side, shallow sleep reduces the recovering side, and a switched-on sympathetic nervous system changes how the body apportions its effort. The three occur in different layers, and at different times: the nose the moment pollen lands, sleep over several days, the sympathetic nervous system throughout the weeks when temperature swings.
Sleep has been measured on this point. In a study where 164 people wore a wrist device recording how much they actually slept over 7 days and were then exposed to a cold virus, those sleeping under 6 hours a night caught colds at a markedly higher rate than those sleeping over 7 hours. With too little sleep, the body answers the same virus differently.
Addressing only one of the three will not make spring easier.
So in spring I suggest going outside
It is light that sets the clock — morning light above all. A short spell of light outdoors in the morning brings the body's clock into step with spring far faster. Indoor lighting does not do it well; the difference in brightness runs to tens of times.
So when someone comes in saying sleep is shallow in spring, I ask about the morning before I ask about the bed. What time they wake and when they first see light is what sets that night.
That said, it is better to avoid days when fine dust is heavy. It adds a burden to a lining that is already thinned. On those days, taking light by a window is enough.
What the body meets differs from season to season. Summer is a hard season for its own reasons, and I have written separately about how the body keeps time.
When to be seen first
If fever continues for several days, if yellow discharge comes together with facial pain, or if the nose is blocked enough to make breathing difficult — being seen comes before any of this.
Korean medicine watched the turns between seasons closely, and set spring down in particular as the time when what was held within rises outward — what winter had gathered in now spreading again.
I find that description interesting every time I read it. Put into today's terms it is the work of moving a regulation set for winter over to spring, and the clock, the mucosa and the autonomic nerves are all caught up in that moving.
The old physicians had reasons for writing each season differently. I check those reasons one by one and put them to use in the consulting room.
Sources
- A study measuring how much people actually slept, then exposing them to a cold virus, and comparing sleep duration against who fell ill — Sleep, 2015
- A review of how light travels through the retina to the suprachiasmatic nucleus and sets the body's clock — Molecular Medicine Reports, 2025
- Allergic rhinitis — the nasal lining's hypersensitive response and its symptoms — Asan Medical Center
Written by Dr. Heo Ji-young (Ph.D. in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)
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