🤒 Why you need to sleep in time with your body clock

During daylight hours, your human circadian rhythm activates brain and body processes designed to keep you awake and alert. But things are dialled down at night, to reduce alertness and aid sleep. This includes your core body temperature.

Body temperature rises throughout the day, peaking late in the afternoon (the best time for maximising athletic performance). Around 6pm temperature starting to decline quickly in advance of sleep.

Core body temperature continues to drop as your near bedtime, bottoming out a few hours after sleep onset. This drop cues important functions, allowing for neuron and nervous system repair, neuron growth, an upregulation of circulating T cells (the killer cells of your immune system), and a decrease in inflammation. If you can get solid sleep during this phase, you’ll have a stronger immune system and less inflammation. But in order for your core temperature to drop like this, you need to have been asleep for a few hours already.

The thing is - your circadian temperature rhythm doesn’t depend on whether you’re actually asleep! Temperature is just one of many circadian rhythms that the suprachiasmatic nucleus governs. If I kept you awake all night, your core body temperature would still rise and fall across the 24-hour period regardless of whether you’ve slept or not!

🚿 Hot or cold showers before bed - and what do they do to your sleep?

While cold water immersion is a hot topic right now - it’s not always best for your sleep. Being immersed in cold water can raise levels of stress hormones involved with boosting alertness levels, in turn increasing heart rate and compromising sleep quality.

That being said - if you’re trying to recover from a bout of intense exercise late in the day, cold showers can still be a useful tool to cool down an already overheated body. One study found that athletes who immerse themselves in cold water for ten minutes after evening exercise experience a drop in core body temperature, fewer nighttime arousals, and a greater proportion of deep sleep within the first three hours of sleep.

This meta-analysis of 17 studies found that taking a “moderately warm” evening shower or bath (in water around 40°C or 105°F) can improve sleep quality - though getting much hotter might be detrimental instead.

While the ancient Indian practice of Ayurveda also recommends against overly hot or cold showers before bed, it also recommends bathing within one hour before sleep. The science seems to back this up - suggesting that carefully timing your bathing can have the added benefit of helping you fall asleep faster:

While experimenting with very hot or very cold showers in the evening can work for the individual, for most people some good rules of thumb are:

• Avoid cold water immersion later in the day, to prevent a stress response before bed.

• Shower in “moderately warm” water (around 40°C) to work in tandem with your body clock.

• Timing showers 1-2 hours before bed can help you fall asleep quicker.

❄️ Using skin temperature & room temperature to sleep deeper

First things first: sleep is strongly linked to temperature regulation at night - and it’s important to work with your circadian temperature cycle.

Using a sauna or exercising too late may interfere with sleep-onset and deteriorate deep sleep. Too hot? You nervous system kicks into action to cool you down. REM sleep drops dramatically, but deep sleep also suffers. Often, people with difficulties staying asleep often have nocturnally elevated core body temperature. So it’s crucial too cool down your body before going into sleep.

But what many people don’t realise is that it’s not just about room temperature, skin temperature can matter just as much too:

Whether it’s a hot or cold time of year, wearing loose and comfortable clothing and using bedsheets made from natural, breathable materials like cotton can help your body disseminate heat to maintain a stable temperature throughout the night.

🔍️ Actionable insight: Find those woolly socks!

The science suggests that keeping your toes toasty can help you fall asleep quicker and wake up less during the night. It’s thought that this caused by “distal skin vasodilation” (keeping you blood vessels open at your extremities), which helps your body regulate your temperature at night.

🛠️ My current Sleepstack:

• Screen filter software: Tools like f.lux (all desktops), Twilight (Android) and Night Shift (iOS) change the colour of your display to based on the time of day to reduce overstimulating wavelengths.

• Sleep mask: I use this one at the moment. Light affects melatonin secretion even when your eyes are shut. A good sleep mask is life-changing, I’m never going back.