Does Sitting By A Window Provide Real Daylight Benefits?

By Bill Chan, President at LiteController Inc.

During talks I’ve given over the years, I often encourage people to take a morning walk outside for at least half an hour—so their eyes are exposed to daylight and their circadian rhythm can stay aligned. However, I am often asked a very practical question:

“If I sit by a window indoors, do I still get the benefit of sunlight for my body clock?”

This becomes especially relevant in Canada during winter. Snow and icy conditions make it difficult—and sometimes unsafe—to spend time outdoors. As a result, many people rely on indoor daylight as their primary exposure.

At the same time, a common concern comes up:

“Does window glass block sunlight and reduce its benefits?”

So instead of answering theoretically, I decided to measure it.

What I measured

I compared two conditions:

• Direct outdoor morning daylight
• Daylight passing through a typical double-glazed residential window (clear glass)

The results were:

• Outdoor vertical illuminance: 7,967 lux
• Immediately behind the glass: 5,718 lux → about 72% transmission
• So most daylight still passes through the window glass.

Then I measured at a more realistic position:

• 1.2 m from the window at approximately eye level (1.2 m above floor)

At that position:

• Vertical illuminance ≈ 1,773 lux

In other words:

Indoor eye-level daylight was still about 22% of outdoor daylight.

So the question becomes: is that enough for circadian stimulus?

What happens to the spectrum?

The spectral measurements as per the image above revealed something important:

• UV (<380 nm) → strongly reduced
• Violet / shorter wavelengths → reduced more
• Visible light (including 460–490 nm) → largely preserved in shape

When normalized to 550 nm:

When comparing relative spectral shape, the blue region (~460 nm) is only slightly reduced, while attenuation increases significantly toward the UV range.

What this means for your body clock

In daylight conditions (≈ D65 spectrum), vertical illuminance at the eye is a good proxy for m-EDI.

So in this case:

• 1,773 lux ≈ 1,773 m-EDI

This is not just sufficient—it is far above the commonly referenced threshold for circadian stimulus of 250 m-EDI.

A quick reality check: eye health vs. wavelength

Different parts of the spectrum affect the eye differently:

• Cataract risk → mainly UV-B (~300 nm)
• Circadian response → mainly blue-turquoise (456-490 nm)

The key point:

Window glass blocks most UV (protective), while preserving the visible wavelengths that drive circadian response.

What really matters in practice

From both measurement and lighting physics, three things matter most:

• Distance from the window
• Viewing direction (light must reach the eyes)
• Sky condition (clear vs. overcast)

It is also worth noting that these measurements were taken with a relatively large window—smaller windows would deliver significantly lower light levels at the eye, even under the same outdoor conditions.

In practical terms, this highlights that daylight design is not only about glass transmission, but also about window size and occupant positioning relative to the opening.

In fact, the combined effects of distance, viewing direction, and window size can be as significant as—or even greater than—the reduction caused by the glass itself.

So… does sitting by the window work?

Yes — with one important condition.

It works — if your eyes are actually exposed to the daylight.

Bottom line

Clear window glass reduces light intensity—but largely preserves the wavelengths that matter.

And more importantly, even at a typical seating distance, daylight can still provide more than sufficient circadian stimulus.

Final takeaway

If you’re indoors during winter:

• Sit closer to the window
• Face the daylight (not just your desk)

Republished with permission.

Image above courtesy of Bill Chan.

About The Author:

Bill Chan is the President of LiteController Inc., a manufacturer of Circadian Lighting LED fixtures and modules. Bill has also been an Honorary Professor of Electrical Engineering at Tongji University for 32 years. Bill lives and works in Markham, Ontario, Canada.

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