Written by Oura in partnership with Zero
How you eat impacts how you sleep more than most people realize. Late night meals can affect the time it takes for you to fall asleep, your sleep efficiency, and how much REM or deep sleep you get.
The reason is more evolution than strictly biology—it goes way back to the cave. Before modern times, humans hunted and ate during the day and slept and rested at night. Over time, our bodies developed networks of internal clocks, including ones in your brain and digestive system (stomach, liver, and pancreas).
If you gobble a hamburger or brownie before you hit the sack at night, your internal clocks get confused. The one in your brain sees that the sun has set and is primed for sleep. The clocks in your digestive system kick into high gear—those organs are now actively digesting instead of relaxing—at the very moment when the other clocks are preparing for bed.
You’ve now forced your body to sleep and digest at the same time—activities that the body has not evolved to do well in parallel. This splits your body’s attention between two tasks, and it struggles to do a great job at either one.
Fasting and Sleep
The clocks in your digestive system don’t enjoy getting activated right before sleep, so fasting and sleep are a complementary pairing. Right?
The answer is yes, and there’s more to it.
If you’re embarking on a serious fasting routine, it’s best to be equipped with a few more details before diving in, because different fasts affect sleep differently.
Circadian Rhythm Fasting
If you eat when it’s light out and fast when it’s dark, your brain and digestive system are naturally aligned. When your brain sends the message to power down, your stomach, pancreas, and liver are already done for the day and agree it’s time to go offline.
If you have a large meal too close to bed, your digestive system votes to prioritize processing food over sleep.
First-Time Intermittent Fasting
Many first-time fasters report disrupted sleep—the dreaded experience of lying wide awake in bed, bored and hungry. Why is this?
Even though it’s dark outside and your body’s other clocks have called for bedtime, the clocks in your digestive tract are reporting back: “We haven’t eaten anything in a while! Are you sure we shouldn’t stay up and look for food?”
As a result, your body might jump into action and produce the stress hormone cortisol to help keep you awake in case food walks by.
This is temporary—your body is simply getting used to a new routine. After an adjustment period that generally lasts 3 to 7 days, your body recaptures its rhythm and fasting can actually benefit sleep.
Long-Term Intermittent Fasting
When you fast regularly, your body adapts to your new schedule and your circadian rhythm becomes more pronounced (in a good way). Intermittent fasting causes insulin levels to drop and melatonin levels to rise. Melatonin is your body’s primary sleep-promoting hormone and can help you fall asleep faster and stay asleep longer.
Fasting also promotes the release of human growth hormone, one of your body’s vital resources for repairs while you’re asleep.
Tips for Sleeping on an Empty Stomach
While you’re sleeping your body fasts naturally. To take advantage of your eating schedule and align your internal clocks:
- Avoid big meals close to bedtime: Reaching for a midnight snack? Think again. Eating elevates your metabolism and heart rate before bed and harms your sleep. It’s best to avoid heavy meals 3 hours before bedtime.
- Take it easy when you start fasting: Higher levels of cortisol when you first start fasting can signal to your body that you are in fight-or-flight mode. Consider taking additional time to unwind and relax your body before bed to promote a restful night’s sleep.
About the Caves, Those Clocks, and Measuring Sleep
Today’s hectic, always-on lifestyle, has resulted in humans being out of rhythm with the circadian instincts we evolved during the cave days. Our bodies simply have not adapted to this new reality of relative food abundance, leading to chronic under-rest or sleep deprivation.
Further complicating matters is the complexity and diversity of the human population. Individuals won’t all react the same way to fasting regimes. This provides an opportunity for objective sleep measurement tools—like Oura—to guide us with insights about how fasting and other behaviors impact us on an individual basis. Oura empowers you to own your inner potential by providing this type of insight so you can make better daily decisions.
Oura tracks your heart’s activity and body temperature throughout the night, which gives you clues into whether your internal clocks are aligned. For instance, you’ll notice a higher-than-baseline heart rate and body temperature on nights when your body is multi-tasking between sleep and digestion.
Conversely, you can also see how fasting helps your internal clocks stay aligned as you track and observe differences in sleep quality between fasting and non-fasting days.
We’re not going back to the cave anytime soon, but we now have the knowledge and tools to get back in rhythm with the incredible biology of the human body.
Zero Editor’s Note:
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