Does cold showering boost your immune system?
Cold showering produces measurable stimulation of the immune system, but whether it actually makes you less likely to fall ill has not been proven. Until better research is available, there is no harm in doing it if you enjoy it, but do not count on it as protection against infections.
After 6 weeks of cold-water immersion (14°C, three times per week), the proportion of certain white blood cells rose slightly but measurably. Antibody levels and total white blood cell counts did not change. What that small shift means in practice for your chances of catching a cold or the flu is unknown.
A single brief cold-water immersion at 14°C caused a sharp rise in stress hormones such as adrenaline and noradrenaline. The effects on the immune system were mixed: lower levels of one inflammatory substance (TNF-alpha), but also fewer lymphocytes (cells that help fight infections). Participants did not fall ill within 48 hours, but 48 hours is simply too short a window to measure that reliably.
The well-known Wim Hof study showed a markedly lower inflammatory response to an experimentally administered bacterial toxin following a 10-day training programme. However, that programme consisted of three components: cold-water immersion, meditation, and breathing techniques. It is impossible to say how much the cold alone contributed. This result therefore cannot simply be attributed to cold showering by itself.
People who report regularly taking cold showers or cold-water swimming report shorter and milder colds via questionnaires. However, these are self-reported data collected online, without any objective measurement of infections. Anyone who voluntarily plunges into ice-cold water differs from those who do not in more ways than one. Cause and effect cannot be distinguished here.
In short: there are biological indications that cold stimulates the immune system, but whether this leads to fewer or shorter infections has not been demonstrated in controlled research. An added complication is that cold can simultaneously provide a mild stimulus to the innate immune response while suppressing the specific (adaptive) immune response, depending on how quickly a person cools down.
Sources: eight studies, two of which have moderate quality of evidence (one randomised study on Ironman recovery, one on the Wim Hof method), the remainder limited or insufficient. Not a single study measures the effect of cold showering alone on the actual occurrence of infections in a controlled design.
What does sleep deprivation do to your immune system?
Sleep deprivation demonstrably weakens your immune system: sleeping fewer than six hours significantly increases your infection risk and impairs your immune cells. Make sure to structurally get seven hours or more, because a single recovery night is not enough to repair the damage.
Does stress really lower your immune system?
Prolonged stress demonstrably lowers your immune system: fewer active immune cells, a weaker response to vaccines and a greater risk of infections. Short-term stress has precisely the opposite effect and makes your immune system briefly more alert.
Does sunbathing boost your immune system beyond the vitamin D effect?
Sunlight has real effects beyond vitamin D through nitric oxide and hormonal pathways, but randomised evidence for improved immune defences is lacking. At the same time, the immunosuppressive effect of chronic exposure is well documented, so moderate sun exposure makes more sense than a great deal of it.
Does your immune system really weaken as you get older?
Your immune system does indeed weaken as you get older, but how quickly that happens varies considerably from person to person. Lifestyle factors play a role in this, although little has yet been proven specifically for humans.
Does too little sleep weaken your immune system?
Yes, too little sleep demonstrably weakens your immune system and increases the risk of infections and chronic inflammatory conditions. Seven to nine hours of sleep is the most commonly cited guideline, and vaccinations also work better when you are well rested.
Does fermented food like kimchi boost your immune system?
Kimchi and other fermented products can improve your gut microbiome and lower inflammatory markers, but whether that actually means you get sick less often has not yet been conclusively proven. Feel free to eat them as part of a varied diet, but do not count on them as a medicine.