Does cold showering boost your immune system?
Cold showering does not demonstrably strengthen your immune system in the short term, but people who do it regularly do report less sick leave. If you want to increase the chance of an effect, combine it with breathing techniques.
Cold showering does not make you directly ill. A study in healthy young men showed that a brief cold immersion does raise stress hormones such as adrenaline and cortisol, but participants did not develop cold symptoms within 48 hours. At the same time, cold immersion triggers a temporary inflammatory response, which is a normal stress reaction of the body.
As for immune markers immediately after showering: they barely change in any measurable way. Eleven studies combined found no significant effect on immune function directly after, or up to one hour after, cold immersion. Smaller studies in athletes did find that six weeks of regular cold immersion led to a modest increase in certain immune cells, but what that means in practice for your health remains unclear.
The most concrete finding about actually getting ill comes from a larger analysis: participants who showered cold regularly had 29% less sick leave. That figure sounds compelling, but it is not a hard measurement. It comes from self-reporting rather than a controlled design, so whether cold showering itself is the cause remains uncertain.
The most striking results came from a small study (24 people) using a combination programme of meditation, breathing techniques and cold immersion. Participants who followed this for ten days showed a clearly less intense inflammatory response when experimentally exposed to a bacterial signal, and reported fewer flu-like symptoms. Cold showering alone was not the only factor here; the breathing techniques very likely played a role as well.
If you want to support your immune system, cold showering on its own is an uncertain tool. There is no evidence that it directly and measurably strengthens your immune system. If you want to make the most of the most promising signals, the combination with conscious breathing techniques appears to have more effect than cold water alone.
This answer is based on an analysis of eleven controlled studies combined, supplemented by a randomised study (24 people), a few small studies in athletes and a questionnaire study. The researchers behind the large-scale analysis themselves emphasised that the evidence for 29% less sick leave does not come from the formal analysis but from self-reporting, and should therefore be interpreted with caution. The evidence for direct immune effects is reasonably solid (no effect), but for actually getting ill less often it is thin and based on self-assessment or small groups.
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.
What does regular sauna use do to your immune system?
Regular sauna use produces measurable changes in your immune system, but whether it actually makes you get sick less often has not yet been proven. The signals are positive enough to regard the sauna as part of a healthy lifestyle, but specific health claims are premature.
Does sunbathing support your immune system beyond the vitamin D effect?
Sunlight influences your immune system beyond the vitamin D effect, through hormones, neuropeptides, and shifts in immune cells. Regular, moderate sun exposure without burning appears to be the most practical way to make use of this effect.
Does your immune system really weaken as you get older?
Your immune system demonstrably weakens with age, but vaccinations remain worthwhile and a good diet can limit further decline.
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.