The short-term health effects of cold exposure
It's quite easy to find webpages describing health benefits from cold showers. Here is an example: 5 Reasons Why You Should Take a Cold Shower | Hawk cHill. Warning: the author has no idea what they are talking about, and you should not click on that link. The study they cite to support benefits for depression presents a hypothesis and proposal for conducting a clinical trial [http:doi:10.1016/j.mehy.2007.04.05], but no results. The study they cite to support benefits for anxiety involved exposure to a 42.0°C spa, which is hot, not cold [https://www.jstage.jst.go.jp/article/biomedres/27/1/27_1_11/_pdf/-char/en].
What is known:
- Twenty healthy male volunteers, placed in a cold room (10°C) for hours showed: Metabolic rate, systolic and diastolic blood pressure all increased. Noradrenaline increased. Cortisol, GH, and prolactin decreased. Pulse decreased. There were no significant changes in adrenalin, T3, T4, TSH, testosterone, or LH. [Leppäluoto, J., Korhonen, I., Huttunen, P., & Hassi, J. (1988). Serum levels of thyroid and adrenal hormones, testosterone, TSH, LH, GH and prolactin in men after a 2-h stay in a cold room. Acta Physiologica Scandinavica, 132(4), 543–548. http://dx.doi.org/10.1111/j.1748-1716.1988.tb08363.x]
- Ten young men immersed in cold water up to their necks for one hour. Water was at one of: 32°C, 20°C, 14°C. Results: Colder water increases diuresis and norepinephrine and dopamine concentrations. Cold immersion activates the sympathetic nervous system, not the HPA axis. [Šrámek, P., Šimečková, M., Janský, L., Šavlíková, J., & Vybíral, S. (2000). Human physiological responses to immersion into water of different temperatures. European Journal of Applied Physiology, 81(5), 436–442. http://dx.doi.org/10.1007/s004210050065]
- What about testosterone? No effect, except daily exposure to extreme cold (-40°C) lowers testosterone. [Pääkkönen, T., & Leppäluoto, J. (2002). Cold exposure and hormonal secretion: A review. International Journal of Circumpolar Health, 61(3), 265–276. http://dx.doi.org/10.3402/ijch.v61i3.17474]
- The diving response (or diving reflex) is triggered by cold water exposure of the upper face (specifically, the ophthalmic division of the trigeminal nerve, so, forehead, eyes, nose). The effect is breath-holding, bradycardia, reduction of blood flow to the limbs, and a rising mean arterial blood pressure. If your upper face is exposed to cold water, then your nose and mouth are also immersed (or you are upside-down). The purpose of the diving response is to divert limited blood oxygen to the vital internal organs. [Gooden, B. A. (1994). Mechanism of the human diving response. Integrative Physiological and Behavioral Science, 29(1), 6–16. http://dx.doi.org/10.1007/BF02691277]
- Acute exposure to cold is a significant cardiovascular stressor for those with cardiovascular disease, and may cause myocardial injury. [Zhang, X., Zhang, S., Wang, C., Wang, B., & Guo, P. (2014). Effects of Moderate Strength Cold Air Exposure on Blood Pressure and Biochemical Indicators among Cardiovascular and Cerebrovascular Patients. International Journal of Environmental Research and Public Health, 11(3), 2472–2487. http://dx.doi.org/10.3390/ijerph110302472]
- The health effects and possible harms (including drowning, hypothermia, cardiac arrest, treatment of inflammation and hyperthermia) are reviewed by Tipton et al. [Tipton, M. J., Collier, N., Massey, H., Corbett, J., & Harper, M. (2017). Cold water immersion: kill or cure? Experimental Physiology, 102(11), 1335–1355. http://dx.doi.org/10.1113/EP086283]
- There may be psychological benefit to overcoming an adversive stimulus through willpower.
Summary: Cold exposure activates the sympathetic nervous system, so you get blast of norepinephrine and dopamine. If cold water hits your upper face, then your parasympathetic nervous system is also activated, and your pulse goes down. Cold shocks may be severe physiological stressors. If you think cold exposure will elevate your testosterone, then you have been submerged for too long.