May 12, 2025, 12:58 pm | Read time: 5 minutes

Despite going to bed early, do you feel exhausted the next morning? This phenomenon might be due to what’s known as “sleep inertia.” Starting your morning with natural light can help alleviate this—such as with curtains that automatically open. A new study indicates that natural light can significantly improve waking up, provided it enters the bedroom at the right time.

What Was Investigated?

The research team at Osaka Metropolitan University wanted to find out how natural light in the bedroom specifically affects waking up–and whether not just the amount of light, but especially the timing, is crucial.

Although many people rely on light alarms or automatically opening curtains, there have been few studies to date that have tested these everyday applications under controlled conditions.¹

Three Different Light Situations in the Test

For the study, a sleep lab was set up like a normal bedroom–with the difference that the windows were equipped with automated curtains. These opened at different times depending on the experimental setup. The three light conditions were labeled as follows:

IA = Intervention A

Daylight was gradually let into the room only 20 minutes before the planned wake-up time.

IB = Intervention B

The curtain opened as early as dawn, much earlier–often 60 to 90 minutes before waking up.

CC = Control Condition

The curtains remained closed, and there was no light before waking up.

All 19 participants (including nine women) slept in the lab for three nights each and went through all three light conditions–in different orders. This allowed the effects to be directly compared.

How Was Waking Up Measured?

To find out how light affects morning well-being, the researchers used both questionnaires (subjective) and physical measurements (objective). This allowed them to capture how awake the participants felt–and how awake they actually were. The following were examined:

• The personal feeling of fatigue immediately after waking up,

• reaction speed in a short screen test,

• brain activity measured with a portable headband,

• physical recovery through heart rate measurement, and

• sleep duration and rest using a motion sensor.

Measurements were taken both immediately after waking up and about an hour later.

Researchers Found: Light 20 Minutes Before Waking Up Works Best

Does more natural light in the morning in the bedroom have a better effect on waking up–or is the right timing more important than the amount of light? The researchers found: The most effective light was when it gradually entered the bedroom only 20 minutes before waking up (Variant IA). In this case, the study participants felt more awake, reacted faster, and were mentally more alert–as shown by both their own assessments and the measured data.

In contrast, when the light was let in from dawn (Variant IB), the results were worse–even though there was more light in the room overall in this group. The explanation: Those who get light too early may wake up too early or restlessly–and end up feeling less refreshed.

The participants felt least refreshed when the room remained completely dark until the alarm (control group). They were measurably more tired and rated themselves as sleepier. This shows: A little light before waking up can help, but not too early and not too much.

The Amount of Light Is Not Decisive

Interestingly, although more light entered the bedroom in the “early light” condition (Variant IB), the waking experience was worse than with light exposure shortly before waking up (IA). Thus, it is not the amount of light that is decisive for a better effect on waking up, but the timing.

Too early or prolonged light can disrupt sleep–especially for people who go to bed late. The light then reaches them in the middle of the deep sleep phase–and this can do more harm than good.

What Does This Mean for Everyday Life?

Those who have trouble getting going in the morning can achieve a lot with a simple measure: let natural light into the bedroom shortly before waking up–ideally about 20 minutes beforehand.

It is important that the light does not come too early–especially if you sleep late. Open east-facing windows or poorly darkening curtains let light into the room too early in the morning. As a possible consequence, you wake up before the alarm and may feel less refreshed.

Limitations of the Study

Although the study was carefully planned and conducted under realistic conditions, there are still limitations:

• The participants were young, healthy, and had a regular sleep rhythm.
• The test period was in winter–with stable weather conditions and clear dawn.
• People with sleep disorders, older individuals, or shift workers were not part of the study.

Whether the results can be transferred to other groups or seasons needs to be examined in further studies.

Conclusion

The study clearly shows: It is not the amount of light, but the timing that is crucial. The participants’ waking experience was best when the light entered the bedroom 20 minutes before waking up. The researchers’ explanation: Light that comes too early can interrupt sleep and impair recovery–too much at the wrong time does more harm than good.

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What Can I Do to Wake Up Better?

This means that a lot can be achieved with simple light control–without artificial light sources. For example, with timers for installed mechanical shutters or an automatic curtain opener that can be attached to existing curtain rods.

A pleasant way to “naturally” wake up with an artificial light source is a daylight alarm clock.