The body's internal clock does not keep exact time. It runs slightly long and is corrected daily by light, which is why light exposure has such a large influence over when sleep arrives.
The clock needs a daily correction
Left without external cues, the human circadian rhythm settles at a period slightly longer than twenty-four hours, so it drifts later each day.
Something has to reset it against the actual day, and the dominant signal for that correction is light reaching the eye.
Without adequate light, the drift continues, which is what produces the gradually later sleep timing observed in people with very low daytime light exposure.
The eye contains a separate light-sensing pathway
Alongside the cells used for vision, the retina contains receptors whose main role is signalling ambient light level to the clock rather than forming images.
These are most responsive to short-wavelength light, which is the part of the spectrum most prominent in daylight.
Because their function is separate from vision, a room can appear perfectly bright while providing far less of the relevant signal than being outdoors does.
Timing determines the direction of the effect
Light in the morning advances the clock, making sleep arrive earlier the following night, while light in the evening delays it.
The same exposure therefore has opposite effects depending on when it occurs, which is the single most useful fact about managing sleep timing.
There is also a period in the middle of the night when light produces the largest shifts, which is part of why interrupted night shifts are so disruptive.
Indoor light levels are lower than they appear
The eye adapts to a very wide range of intensities, so subjective brightness is a poor guide to the amount of light actually being received.
Outdoor illumination even on an overcast day is substantially greater than typical indoor lighting, which means time outside is not readily substituted indoors.
This is why morning outdoor exposure is emphasised in sleep advice, and why the recommendation is about being outside rather than about brighter rooms.
Evening exposure matters more for its timing than its source
Screens receive most of the attention, though ordinary household lighting in the hours before bed contributes a considerable amount as well.
Reducing overall evening light levels tends to be more effective than filtering a single device while leaving the room brightly lit.
Anyone whose sleep timing is severely displaced, or who works nights, is dealing with a clinical situation where general advice is less useful than assessment by a doctor.