diff --git a/PRINCIPLES.md b/PRINCIPLES.md
index 85341eb..071f47f 100644
--- a/PRINCIPLES.md
+++ b/PRINCIPLES.md
@@ -267,3 +267,4 @@ See also [Calm programming / Slow programming](README.md#calm-programming--slow-
 * Do one thing at a time. Only one item under your name in the WIP. The rest will wait. 
 * Stay positive. Focus on what is doing ok, what you have accomplished. Focus your brain attention more often on something that is stress free.
 * Limit your coffee intake. Drinking caffeine triggers the release of adrenaline. Adrenaline is the source of the “fight-or-flight” response, a survival mechanism that forces you to stand up and fight or run for the hills when faced with a threat. The fight-or-flight mechanism sidesteps rational thinking in favor of a faster response. This is great when a bear is chasing you, but not so great when you’re responding to a curt email. [source](https://www.linkedin.com/pulse/20140805002649-50578967-how-successful-people-stay-calm/)
+* Sleep. When you sleep, your brain literally recharges, shuffling through the day’s memories and storing or discarding them (which causes dreams), so that you wake up alert and clear-headed. Your self-control, attention, and memory are all reduced when you don’t get enough—or the right kind—of sleep. Sleep deprivation raises stress hormone levels on its own, even without a stressor present. Stressful projects often make you feel as if you have no time to sleep, but taking the time to get a decent night’s sleep is often the one thing keeping you from getting things under control. [source](https://www.linkedin.com/pulse/20140805002649-50578967-how-successful-people-stay-calm/)