Cold water swimming is currently considered as a treatment for Major Depressive Disorderplugin-autotooltip__plain plugin-autotooltip_bigMajor Depressive Disorder

Major Depressive Disorder (MDD), also known simply as 'depression', is a mental disorder characterized by at least two weeks of pervasive low mood.

Current estimates are that severe depression affects around 2% of the global population at any given time, currently more than 150 million. The majority of cases don't appear to have an obvious personal and/or societal cause.. Several studies have examined the possible benefits, but results are not clear, partly because of the problems in excluding placeboplugin-autotooltip__plain plugin-autotooltip_bigPlacebo effect

Placebos are faux medications - typically lactose or microcrystalline cellulose - designed to act as inert, non-functional 'controls' in medical experiments. (There are also faux medical procedures, see section below)

From the beginnings of their use in research, experimenters had assumed that they could not possibly have any medical effect. But they are now being taken seriously as treatments, after it was discovered that totally inactive substances effects, and social / lifestyle changes.

Whether or not there are benefits - and many proponents assert that there are - the exact biological and psychological mechanisms are not currently known.

An example paper :

Exposure to cold is known to activate the sympathetic nervous system and increase the blood level of beta-endorphin and noradrenaline and to increase synaptic release of noradrenaline in the brain as well."

Source : Medical Hypotheses, ^(paywalled) Volume 70, Issue 5, 2008, Pages 995-1001

Also see : a case report : Open water swimming as a treatment for major depressive disorder ^(paywalled) BMJ, Vol. 2018

Another study found that although cold water swimming gradually improved general mood (and helped with some physical disorders such as rheumatism) it also led to more referrals to doctors. See :Winter swimming improves general well-being International Journal of Circumpolar Health, Volume 63, 2004 - Issue 2

Editor's Note: There seems to be a lack of recent scientifically controlled studies into the subject.