Scientists tested three wake-up routines. One type of bedroom lighting clearly stood out.

Many people struggle with grogginess in the morning, a common experience known as sleep inertia. A new study published in Building and Environment suggests that a simple change to your bedroom lighting might help. Researchers from Osaka Metropolitan University found that exposing sleepers to a moderate amount of natural morning light—specifically starting 20 minutes before waking—led to improved alertness and reduced sleepiness compared to no light or longer light exposure starting at dawn. The findings suggest that carefully timed exposure to natural light could be a practical, energy-efficient way to enhance morning wakefulness.

The researchers were interested in finding better ways to help people transition from sleep to wakefulness, especially in modern environments where many sleep indoors with limited exposure to natural light. While previous research has focused heavily on artificial lighting—such as dawn simulation devices that mimic sunlight—this team wanted to explore whether natural light might offer more effective or more realistic benefits in everyday life. Given that people’s biological clocks evolved under natural lighting conditions, the team hypothesized that natural light, when appropriately timed and controlled, could improve the process of waking up.

To test this, the researchers designed a controlled laboratory experiment that mimicked an ordinary bedroom environment. The study was led by Xiaorui Wang, a graduate student in the School of Human Life and Ecology, and Professor Daisuke Matsushita. Nineteen university students aged 20 to 30 participated in the study. Each participant stayed in a specially outfitted sleep lab on three separate nights. The researchers used motorized curtains to expose participants to different types of natural light before waking.

The experiment tested three conditions. In one condition (called IA), participants were exposed to natural light for 20 minutes before their scheduled wake-up time. In another condition (IB), participants received natural light from dawn (when the sun was just below the horizon) until waking. In the third condition (CC), participants received no natural light before waking. The same participants experienced all three conditions, one each night, in a rotating order to reduce bias. This design allowed the researchers to compare the effects of the lighting conditions on the same individuals, which helps control for personal differences in sleep habits or physiology.

After waking, the researchers measured participants’ sleepiness, alertness, and fatigue using both self-report and biological tools. Subjective sleepiness was assessed using the Karolinska Sleepiness Scale, and alertness was measured with a reaction time test known as the Psychomotor Vigilance Test. For objective data, the researchers recorded brainwave activity with electroencephalograms (EEGs), and heart rate variability with wearable sensors to assess physiological fatigue.

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The results showed that natural light exposure had a measurable effect on awakening quality. Both light exposure conditions (IA and IB) were associated with less subjective sleepiness than the no-light condition. However, the 20-minute pre-wake exposure (IA) was most effective overall. Participants in the IA condition had significantly lower objective and subjective sleepiness and responded faster in the alertness test compared to when they received no light. The IB condition, which exposed participants to light starting at dawn and potentially for a longer period, also helped reduce subjective sleepiness and improve alertness—but to a lesser extent than IA.

Interestingly, the longer duration of light exposure in the IB condition did not enhance wakefulness and in some cases may have been less helpful. The researchers suggested that early or prolonged light exposure might lead to more frequent awakenings during sleep, reducing total sleep time and affecting how refreshed participants felt. Indeed, participants in the IA condition slept slightly longer in the final 90 minutes before waking compared to those in the IB condition.

Fatigue levels, as measured through heart rate variability and self-report, did not show consistent differences across the three conditions. This suggests that while light can help people feel more awake and alert, it may not directly reduce feelings of physical tiredness or physiological fatigue in the short term. However, there was some indication that increased natural light exposure was associated with lower fatigue-related indicators, though these findings were not statistically strong enough to draw firm conclusions.

The study also explored how the amount and timing of natural light exposure influenced outcomes. Measurements of illuminance—the brightness of the light—confirmed that the IA and IB conditions both significantly increased light levels in the bedroom compared to the CC condition. However, there was no meaningful difference in brightness between IA and IB, highlighting that duration and timing, rather than brightness alone, may be more important for waking quality.

While the findings suggest practical benefits of morning natural light, the researchers noted several limitations. The participants were young adults with relatively consistent sleep patterns, and all had late bedtime habits typical of college students. These results may not generalize to older adults, people with sleep disorders, or individuals with different routines. In addition, while the controlled lab environment helped isolate the effects of light, it does not capture the full complexity of real-world sleep environments, which vary in terms of noise, temperature, and distractions.

The researchers also pointed out that the study did not include a washout period between conditions, which might have affected the results slightly. Although the crossover design helped minimize individual differences, some carryover effects from the previous night’s condition could not be completely ruled out.

Despite these limitations, the study makes an important contribution to understanding how natural light can support better sleep-wake transitions. By showing that just 20 minutes of light exposure before waking can improve how people feel and function in the morning, the research points to practical changes that could be implemented in everyday bedrooms using automated curtains or smart lighting systems.

Future research will need to explore how these findings apply across different age groups, housing types, and seasons. The team also hopes to examine how other properties of light, such as spectrum and direction, interact with timing and intensity to affect waking. These insights could help guide the development of more responsive and personalized lighting systems that support human health and well-being.

The ultimate goal is to help people wake up feeling better by working with, rather than against, the rhythms of the natural environment. By aligning light exposure with the body’s internal clock, simple changes to how morning light is managed in the bedroom may offer a low-cost, energy-efficient way to ease the daily transition from sleep to wakefulness. “In the future, we hope to control natural light in the sleep environment as it changes with the seasons and time of day, and to clarify how to introduce natural light that is suitable for a more comfortable awakening,” Matsushita said.

The study, “Natural light control to improve awakening quality,” was authored by Xiaorui Wang, Yangcheng Gu, Jihui Yuan, and Daisuke Matsushita.