More Daytime Light = Better Nighttime Sleep

CHARLIE JARBOE & MARIANA FIGUEIRO Lighting Research Center, Rensselaer Polytechnic Institute

How can lighting help us during this pandemic?

BACKGROUND

Light is the major synchronizer of circadian rhythms to the local position on Earth, essentially telling the body what to do and when to do it. Cycles of sleeping and waking, feeding and fasting, the regulation of core body temperature, blood pressure, and the secretion of hormones are just a few examples of circadian rhythms. Bright light (> 500 lux) received at the eyes early in the day exerts the strongest influence on adjusting (or entraining) the timing our circadian system to regulate these cycles throughout the 24-hour day.

Before the introduction of electric lighting, humans were exposed to bright days and dark nights. Perhaps because we evolved under this strong day-night contrast, we have adapted to our environment with a circadian clock that needs much higher light levels to be activated than the low levels of light required for our visual system. For example, when we sit around a campfire at night, our night vision permits us to easily see each other’s faces, but the campfire does not cast enough light to disrupt our circadian clock.

People in modern society spend more than 90 percent of their time in buildings, yet indoor lighting is usually not bright enough to stimulate the circadian clock. For example, typical indoor lighting provides less than 100-200 lux at the eye, whereas being outdoors on a sunny day will provide anywhere from 1,000 to greater than 50,000 lux at the eye. Yet many of us, including those living in more-controlled environments such as nursing homes and prisons, do not have free or ready access to the outdoors. The situation grows worse for almost everyone during the winter, when we go to work in the dark, remain in a dimly-lit office or classroom all day, and return home in the dark at the end of the workday.

Simply put, most people do not get enough light during the day and are exposed to too much light in the evening, when our bodies should be winding down for a good night’s sleep. Many people, after not getting enough bright light during the day, compound the problem by using self-luminous electronic devices like smartphones and tablets in the evening or stay up late working on the computer. Light from these screens can make the brain’s circadian clock think it’s time to wake up rather than get ready for bed.

Disruption of the 24-hour rhythm of light and dark affects every one of our biological systems, from DNA repair in single cells to the pineal gland’s production of melatonin (the “darkness hormone”). Circadian disruption is most obviously linked with disruption of rest–activity patterns, which can cause sleepiness during the day and insomnia at night, but it is also linked with increased risk for psychological ailments such as anxiety, stress, and depression, and negative physical health outcomes such as diabetes, obesity, cardiovascular disease, and certain types of cancer.

During the COVID-19 pandemic, many of us were compelled to quarantine and work at home. The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute investigated the impacts of working from home on individual daily light exposures, and how this may be affecting sleep quality and psychological health.

METHODS

In May 2020, the LRC invited people who had been staying home due to the pandemic to complete a short survey about their sleep, mood, and daily light exposure.

A total of 708 individuals responded to the survey, 593 of whom were either employed and working from home or were unemployed but quarantining at home during the two weeks prior to completing the survey. Only results from those 593 were included in the analysis. Forty-five percent of the respondents declared themselves male, 51 percent declared female, and four percent declared “other.” Their average age fell within the 45–54 years age group.

The survey was announced via e-mail and releases on lighting industry websites and in the general media. Respondents accessed the survey via a link embedded in the e-mails and media releases, directing them to an electronic survey facilitated by SurveyMonkey. All respondents were informed that their answers would be anonymous, no identifying information would be collected, and their participation granted consent for inclusion of their response data in the analysis.

RESULTS

The results showed that daily indoor light exposure (Figure 1) and time spent outdoors (Figure 2) had a major impact on all survey outcomes, including sleep disturbances, sleeprelated impairment, anxiety, stress, depression, and mood. Compared to those with “somewhat dim” to “very dim” indoor lighting, respondents with “somewhat bright” to “very bright” lighting—provided by either uncovered windows or robust electric lighting—reported:

• Fewer sleep disturbances

• Less anxiety and depression

• Feeling less tired or less irritable

• Feeling generally happier and more positive

• Less sleep-related impairment

Figure 1. The effects of at-home light exposure on the survey outcomes. The error bars represent standard error of the mean.

Another especially important factor was the amount of time respondents spent outdoors. The survey revealed that people who spent one to two hours outdoors each day reported feeling significantly less depression, while reporting better sleep than those who spent less than 30 minutes outdoors each day. Notably, these positive impacts leveled off after durations of two hours outdoors. Consistent with the entrainment hypothesis, morning light provided the greatest benefits for the survey outcomes.

Figure 2. The effects of time spent outdoors on the survey outcomes. The error bars represent standard error of the mean.

The strong trends toward improvement evident in the survey outcomes demonstrated that increasing overall light exposures and spending more time outdoors during the day can help to establish a robust 24-hour light-dark pattern that provides significant positive impacts on our sleep and mood, even during trying times.

Using the power of light to sleep better & feel better

Good sleep is essential for good health, and may even have a protective effect against COVID-19 because a healthy, regular sleep pattern promotes a strong immune system. We recommend the following tips to help you sleep better at night and feel better every day.

• Seek light during the day, especially in the morning.

• If the sky is clear and the sun is shining, go for a 30-minute walk or run every morning at the same time. Morning light provides the greatest health benefits.

• If you must stay indoors during the daytime, work facing a window. Open the window curtains or shades to let in daylight. Remember to keep ambient room lighting turned on during the day to reduce glare from the window.

• If you don’t have a window in your workspace, add more lighting fixtures. For example, if you have only a single table lamp near your desk, add three additional lamps to the space to deliver at least 350 lux of 3000 K or 300 lux of 5000 K light to the eye. Don’t forget to turn off the extra lights later in the day and through the evening.

• Provide an extra boost to your circadian system early in the day by using a DIY circadian-effective desktop luminaire.

• In the evening, use warm, low-level, dim lighting and turn off your self-luminous displays one to two hours before bedtime. The intense glow from the screen can make it harder for you to fall asleep.