As part of our Biohacking Masterclass Series, we will explore a number of different techniques and practices to improve your daily performance. This series will range from obvious subjects like exercise and nutrition to fringe subjects like breathing techniques and sleep hygiene. Consult your physician before implementing any of these topics. We are not your physician. Note: This post may contain affiliate links to the products we use.
The first steps to restorative sleep are not complicated, yet they are undoubtedly underutilized. There’s no need for witchcraft or a chemically-induced stupor. By paying attention to the biological systems that humans have evolved, you can unlock the keys to better sleep.
We’ve discussed one of these systems at length: the light you see at certain times of the day. Circadian rhythms heavily rely on the intensity and hues of light, so we’ve equipped you with the knowledge to take advantage and get more hGH, higher quality sleep, and better recoveries with this nighttime ‘hack’ and this morning ‘hack’.
In this article, we’re exploring an equally (if not more) important environmental sleep cue: how the temperature of your bedroom affects your sleep.
To achieve optimal sleep, your body needs to drop its core temperature. [1-5] Typically, researchers suggest your body must cool by as much as 2-3 degrees Fahrenheit.
This drop in core body temperature is triggered by a decrease in ambient (external) temperature. As external temperature decreases, your body flushes heat to the periphery of your body (your skin). When external temperature is much lower, the heat in your skin is effectively pulled away, allowing your core temperature to decrease.
Today, this process is often complicated by the static (or nearly static) temperatures created by air conditioning and heating. Many households keep their thermostat set to a consistent temperature, often around 72 degrees Fahrenheit. Not only is the consistent temperature prohibitive to normal circadian rhythms, the optimal sleeping temperature is between 60-67 degrees Fahrenheit.
When external conditions are too cold, one can easily adapt by adjusting bedding. When external conditions are too warm, mental and physical recovery face a host of barriers.
What happens when your sleeping environment is too warm?
High external temperatures inhibit your circadian rhythms. Several studies have evaluated the effects of elevated sleeping temperatures, including the impact on certain stages of sleep, nervous system activity, heart rate, and hydration. [1-3]
One such study compared nights with increased temperature to ‘recovery’ nights. When participants were subjected to elevated external temperatures, core body temperature failed to drop from its typical 37 degrees Celsius (98.6 F). Participants saw “less total sleep time, more frequent and longer awakenings, greater shifting among sleep stages,” decreased amounts of restorative sleep stages, and delayed onset of deep sleep. Further, participants that were subjected to heat lost an average of 1.25 kg of fluid. 
If you are sleep deprived or have suffered from a short night of sleep, high external temperatures inhibit your body’s “recovery” sleep. To demonstrate this, researchers subjected two groups of participants to a period of sleep deprivation. The groups were then allowed to recover in normal and high-temperature conditions for subsequent nights. When heat was added, there was no sign of recovery from the sleep debt. Under lower temperatures, there was a significant increase in stage 4 NREM sleep, also known as deep or SWS sleep.  This is particularly relevant for physical recovery, as more than 70% of hGH is released during this stage.
Numerous studies have demonstrated similar detrimental effects. A review of the literature found that “wakefulness is the only stage that can cope with an increased thermal load… and that wakefulness replaces SWS and REM to maintain homeothermy.”
In other words, elevated sleeping temperatures prevent your body from focusing its energy on restoration. Instead, high external temperatures increase wakefulness in order to maintain proper body temperature.
Researchers suggest that after ambient temperature, humidity plays the next most crucial step in climate regulation. While decreased humidity allows body heat to be wicked away with sweat, humid heat exposure limits the amount of moisture that can leave the skin. Therefore, core body temperature remains elevated. 
What are some steps you can take before bed?
If you have access to air conditioning or heating, set your thermostat to drop between 60-67 degrees Fahrenheit whenever you typically prepare for bed. If possible, maintain a humidity of 40-60%. 
Beyond adjusting climate, you can manually force your body to drop its temperature. The degree of heat loss you experience (aka distal-to-proximal skin temperature gradient, DPG) was shown to be the best predictor variable for sleep-onset latency . Put simply, the amount of heat you are able to shed is one of the best predictors for how long it takes to fall asleep. The more your core body temperature drops in the 2-3 degree window, the faster you will be able to fall asleep. One of the best ways to manually decrease core body temperature is with a 30 min.- 1 hour hot bath at least 30 minutes prior to your desired sleep time. A similar effect may be obtained in a dry sauna as well. For a complete guide to Finnish style dry sauna bathing, read our breakdown here.
- Okamoto-Mizuno et al. Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology. 2012. 31(1):14.
- Bach et al. Effect of continuous heat exposure on sleep during partial sleep deprivation. Sleep. 1994. 17(1):1-10.
- Karacan et al. Effects of high ambient temperature on sleep in young men. Aviat Space Environment Med. 1978. 49(7):855-60.
- Krauchi et al. Functional link between distal vasodilation and sleep-onset latency? Am J Physiol Regul Integr Comp Physiol. 2000. 278(3):R741-8.
- Gilbert et al. Thermoregulation as a sleep signalling system. Sleep Med Rev. 2004. 8(2):81-93.