Take cold showers only if you like them
The evidence is thinner than the enthusiasm by a wide margin.
Do this
If you enjoy it or find it useful as a morning cue, carry on. Do not expect documented improvements to immunity, metabolism or mental health, and do not feel you are missing something if you skip it.
Why it works
One Dutch trial found routine cold showering was associated with fewer self-reported sick days, though not with fewer illnesses. Beyond that, studies are small, short, often unblinded and inconsistent. Cold water immersion after resistance training may actually blunt strength and muscle adaptation.
Where it fails
Blinding is impossible, so expectancy effects are large and unmeasured. Cold exposure carries real cardiac risk for some people. The gap between the online claims and the published evidence here is one of the widest in this whole collection.
How you’ll know
If the only benefit you can name is that it feels bracing, that is a real benefit and also the whole of it.