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What light does the eye receive to set the body clock?

The eye is not only for seeing. The retina has cells called ipRGC that contain melanopsin and tell the body's internal clock (circadian rhythm) whether it is day or night. The highest sensitivity of these cells is around 480 nm, in the blue range. 480 nm that is, in the blue range.

The action spectrum of non-visual light, including setting the body clock, is in the range of 447 to 484 nm and has the greatest effect; in other words, its peak is in the blue. At night, the same blue light causes the largest drop in melatonin (the sleep hormone) secretion, which is why night-time blue light harms sleep the most.

Blue light in the day is good, at night it is not

During daytime hours, light with enough blue content helps with alertness, concentration and setting the body clock. That is why whiter, cooler light suits offices and hospitals during the day. In the evening and at night, however, the same light stops the body from "hearing" the signal to sleep early.

CIBSE and SLL guidance (Integrated Lighting Position Statement, 2026) proposes these practical limits, measured by melanopic light intensity (melanopic EDI) at eye level:

The key point is that colour temperature alone does not show how much blue light a source contains; two sources with the same colour temperature and the same lux can have very different melanopic content. That is why CIE and CIBSE recommend specifying the melanopic index in design, not just the kelvin value.

Cornea and lens: what we need to know

The cornea and lens absorb part of the light before it reaches the retina. Based on measurements by Ambach et al. (1994), the cornea almost completely absorbs ultraviolet light; its transmission reaches 1% at about 291 to 298 nm. The lens also absorbs strongly at about 389 to 414 nm, at the violet–blue edge. So most blue light (from about 440 nm upward) passes through both, and the lens mainly reduces the short-wavelength blue edge.

That the lens becomes yellower with age and blue transmission decreases is a known phenomenon, but this article does not yet cite a confirmed source for it.

The absorption figures come from the study by Ambach et al. (1994). Age-related change in lens transmission was not examined in that source and should be checked against a separate source before use in an official report.

Suitable light for each space

The colour temperature and CRI figures in this table are practical suggestions; they do not come from a specific hospital standard, and an official source for hospital colour temperature has not yet been found. Before installation, check them against the local standard or the official text (such as EN 12464-1).

Colour temperature, CRI and glare: three numbers you should know

Colour temperature (K) Describes the feel of the light; the higher the number, the whiter and cooler the light. CRI Shows how true the light renders colours; about 80 is recommended for general use, and 90 and above for spaces where colour matters. Glare Occurs when a direct light source is in the field of view; beam angle, mounting height and the use of a bowl or diffuser reduce it.

The practical point is to choose all three together: cool light with low CRI makes colours look dull, and warm light with high glare makes the space harsh and uncomfortable.