Chapter 2. First Light

From Perfect Day

Go there. It is not yet light and you are awake before the thing that was going to wake you. In about twenty minutes something will ask for you. This chapter is what to do with the interval, in numbers, and why the numbers matter more on one side of forty-five than the other.

Loads carried: thermal (the cold room). Plus the anchor that sets every other load’s timing.

The prescription

Light, by the international consensus, stated in the unit the consensus uses. The measure is melanopic equivalent daylight illuminance — light as the clock’s own receptors see it, weighted to the wavelengths they respond to — measured at the eye. Three figures, from eighteen circadian scientists writing on the basis of an internationally balloted standard:

Daytime: at least 250 melanopic lux at the eye, all day, measured vertically at seated eye height. [Brown TM et al., PLoS Biol 2022;20(3):e3001571, doi:10.1371/journal.pbio.3001571.]
Evening, the three hours before habitual sleep: no more than 10 melanopic lux. [Brown 2022.]
Night, in the bedroom: as dark as practicable, no more than 1 melanopic lux, and never above 10. [Brown 2022.]

The book translates those three numbers into one rule: bright day, dim evening, dark night — and it puts the first of the three in the first hour, because the same 250 lux does more for the clock in the morning than at any other time of day. The ordinary indoor environment fails the daytime figure: common white light sources are inefficient at the wavelengths the clock reads, so a room that reads 300 lux to a light meter can deliver far less to the clock. [Brown 2022, the worked example in its Figure 1.] Outside, even under cloud, the figure is met many times over. [Field measurements: outdoor median about 1,200 lux against an indoor median of about 180; an open playground about 9,300 with a range to over 90,000; a sunny outdoor median roughly 2.4 times the cloudy one — Bhandary SK et al., PLoS One 2021;16:e0254027, doi:10.1371/journal.pone.0254027. Photopic lux, not melanopic; Plausible for the order of magnitude, which is all the sentence needs.]

Timing. The same wake time every day, which Chapter 16 prescribes on mortality data, and light at the eye within the first hour of it. The morning is where light advances the clock and sets the day’s timing; the same light in the evening delays it. [Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA, J Physiol 2003;549:945–952, doi:10.1113/jphysiol.2003.040477: light before the body’s temperature minimum delays the clock by up to about 3.6 hours, light after it advances by up to about 2 hours, in a laboratory pulse of 6.7 hours — Solid for the shape; the ten minutes outdoors is a far smaller dose and the book does not claim the laboratory magnitude for it.]

Temperature. The room slept cool and the first minutes spent in it before the house is warmed. The book gives no bedroom temperature, because the evidence base for a figure is thin. [Okamoto-Mizuno K, Mizuno K, J Physiol Anthropol 2012;31:14, doi:10.1186/1880-6805-31-14: with bedding, sleep is best in roughly the 16 to 19 °C range; humid heat increases waking and cuts deep sleep. Small studies; Plausible for cool, not cold; no single number is proven and the book prints the range as a range.] The thermal load of a cold room is stated as mechanism, not outcome.

Not the phone. The screen is a source of evening-grade light in the morning hour and a source of somebody else’s demand; the second reason is the stronger one and is argued in Chapter 3.