top of page
  • Writer's pictureTimothy P. Smith MS, RD, LDN

Sleeping in Light;"Fight or Flight"?

Updated: Dec 19, 2022

Sleep is a mystery. Nobody really understands why we do it.

Think of how strange it is: The majority of us are awake for 14-18 hours each day, and can't help but fall asleep when it gets dark. Why is it that we need this respite, despite even our best efforts not to? Why don't we just stay awake for 24 hours a day?

"I think we can agree: Sleep is a strange phenomenon that we have virtually no say in."

Some of the most popular theories explaining why we need sleep:

Information Consolidation Theory claims that we sleep in order to more efficiently & thoroughly process information from the previous day, while also priming the brain to absorb information in the following day. [1]

Evolutionary or "Adaptive" Theory claims that we tend to sleep at night because being awake at night is risky for humans, from a predatory standpoint. This is a less commonly accepted theory, but many do still entertain it on an evolutionary basis. [2]

Energy Conservation Theory states that we sleep as a way of conserving energy, as energy from food has not always been a guarantee. So, decreased energy expenditure during sleep means we require less energy from food, potentially staving off starvation under conditions of increased caloric expenditure. [3]

Repair & Restoration Theory attributes sleep requirements to our body's necessity to restore proper physiological/cognitive function via NREM & REM sleep. Basically, our bodies are able to "housekeep" while we sleep, instead of trying to clean up while we're awake. This includes rebuilding muscle tissue after intense workouts, or recovering from injury/illness. [2]

The Clean-Up Theory states that we sleep in order to clear our toxic compounds (such as adenosine) in our brain that build up as a result of being conscious & awake. [4]

There are other theories, but regardless of which you may subscribe to, I think we can agree: Sleep is a strange phenomenon that we have virtually no say in. The longest time any person in recorded history has gone without sleep is approximately 11 days. However, significant health issues would begin to arise for most people after just 2-3 days with your eyes open.

"The longest time any person in recorded history has gone without sleep is approximately 11 days. However, significant health issues would begin to arise for most people after just 2-3 days with your eyes open."


So, if sleep is so important, how can we ensure quality sleep? A March 2022 study from Northwestern University's Feinberg School of Medicine [5] may help us find out. In particular, the two key questions proposed by this research were: "How does sleep quality in a room that is mostly dim, compare to sleep quality in a room that is mostly lit? And what are the health outcomes of sleeping in each environment?"


Study Design

To start, (20) young, healthy human subjects between the ages of 18-40 (average 27 years) were recruited to sleep two nights in the sleep facility at Northwestern University.

These (20) were divided into 2 cohorts of (10) individuals, with the study being conducted as follows:

Cohort 1, Night One: Sleep in dim light (DL)

Cohort 2, Night One: Sleep in moderate light (ML)

Cohort 1, Night Two: Sleep in moderate light

Cohort 2, Night Two: Sleep in dim light

(Not a massive study, but a supplement to the existing literature.)

During this study period, a number of metrics were measured, including:

-Polysomnography (Measures sleep quality, among a number of other metrics)

-Insulin resistance (via HOMA-IR, Matsuda Insulin Sensitivity Index)

-Trends in blood glucose during sleep/wake cycle (Oral Glucose Tolerance Test; eat glucose & see response in glucose/insulin after 30-120 minutes)

-Heart rate/heart rate variability

-Blood pressure

-Epworth Sleepiness Scale (Helps determine sleepiness during the day)

TL;DR: Researchers were looking at markers of sleep quality, cardiovascular health, as well as the body's ability to remove sugar from the blood by the action of insulin.

Key Findings

Upon study completion & analysis, it was found that sleeping in the ML condition reduced the percentage of time in REM sleep ("Deep Sleep"; crucial for health/metabolism.)

In addition, being exposed to moderate light during sleep increased measures of insulin resistance in the following morning, when compared to sleep in the dim light condition. Further, it appears the initial increase in post-OGTT insulin was more dramatic in those exposed to moderate light. This is important because such a harsh response is tied to risk of developing type two diabetes.

It was also noted that heart rate was significantly higher in those sleeping under ML, which indicates increased activation of the sympathetic nervous system ("Fight or Flight"), potentially as a result of sleep disruption.

Interpretation & Potential Application

We can be confident based on the results of this small study that sleeping in dim light is superior to sleeping in a moderately-lit setting. Not to negate the benefits of sleeping in total darkness; a variable not analyzed in this study.

These results are especially relevant to those who are increased risk of developing type two diabetes, or those with an existing diagnosis of diabetes. In particular, the dramatic early increase in insulin following exposure to moderate light reflects compensation in response to insulin resistance. Basically, all measures indicate that sleeping with the lights on can increase activation of the sympathetic ("Fight or Flight") nervous system, which results in reduced sleep quality/recovery, as well as increased insulin resistance (Which is bad.)

So, what should we do?

  • Ideally, sleep with the lights off.

  • If you need a light, use a nightlight with a dim light, out of your direct line of sight.

  • If you tend to watch TV in the evening, use your sleep timer!

  • Consider using an eye cover to help keep light off your eyelids during sleep.

  • If you do have a remarkably poor night's sleep, you might want to avoid having a super sugary breakfast the next morning.

Until next time,

Tim Smith MS, RD, LDN




1.) Born J, Wilhelm I. System consolidation of memory during sleep. Psychol Res. 2012;76(2):192–203. doi:10.1007/s00426-011-0335-6

2.) Ezenwanne E. Current concepts in the neurophysiologic basis of sleep; a review. Ann Med Health Sci Res. 2011;1(2):173–179.

4.) Xie L, Kang H, Xu Q, et al. Sleep drives metabolite clearance from the adult brain. Science. 2013;342(6156):373–377. doi:10.1126/science.1241224

5.) Mason, I., Grimaldi, D., & Reid, K., et al. (2022, March 14). Light exposure during sleep impairs cardiometabolic function | PNAS. PNAS. Retrieved May 4, 2022, from

50 views0 comments


bottom of page