Take a look at the following graphic:
This is from the Medscape CME article “The Impact of Sleep Deprivation on Hormones and Metabolism“ which was published in April 2005 (click on the link in Google Scholar). It’s a bit technical in some parts, but it’s a very good overview of how sleep deprivation affects the pituitary-dependent hormones.
I have previosuly discussed the impact of sleep on human growth hormone (HGH) levels. This piece will focus on the role that sleep plays in the production of thyroid-stimulating hormone (TSH) and how this influences your performance in open water swimming.
If you take a look at the top series of graphs, you can clearly see how sleep deprivation significantly lowers the levels of TSH produced in your brain. TSH regulates the endocrine function of the thyroid gland and stimulates the thyroid gland to secrete the hormones thyroxine (T4) and triiodothyronine (T3). Consequently, insufficient sleep leads to lower levels of thyroid hormone production – something known as hypothyroidism.
While this type of hypothyroidism is mild and temporary, it can nevertheless have a substantial impact on your ability to function in open water environments. This is because one of the primary effects of hypothyroidism (be it temporary or chronic) is cold intolerance.
In other words, even mild sleep deprivation can negatively impact your ability to swim in colder water.
I can tell you from first hand experience that this is right on the mark. And I can also relate that the colder the water temperature, the more rapidly you lose the ability to effectively thermoregulate if you are sleep deprived. When we were out in the lake with water temperatures in the sub-40F range, that one half hour less of sleep made a huge difference in one’s comfort level in the water.
So make sure you rest up properly the evening before any swim in the lake (or any open water event for that matter). Because one half hour of sleep in either direction can make or break you in the water.