The Circadian Window

The human circadian system is anchored by two primary light events: the blue-rich light of morning, which suppresses melatonin and initiates alertness, and the amber light of evening, which signals the body to prepare for sleep. Mariana Figueiro and her colleagues at the Lighting Research Center have demonstrated that morning light exposure above 500 lux significantly improves nighttime sleep quality and reduces daytime sleepiness, particularly in adults over 60. But morning light alone is not sufficient — Duffy and Czeisler's work at Harvard Medical School shows that the *timing* of light exposure matters as much as its intensity. The body needs the contrast between bright morning and dim evening to maintain a stable 24-hour rhythm.

Most bedrooms violate this principle by default. A west-facing bedroom receives evening light but misses the sunrise; an east-facing bedroom catches the morning but loses the dusk. In either case, the occupant wakes or sleeps against the grain of the light they receive. The WELL Building Standard v2 recognizes this problem: its Circadian Lighting feature (L03) recommends providing at least 200 equivalent melanopic lux at the eye during morning hours and minimizing bright light in the evening. But the standard focuses primarily on electric lighting. The more elegant solution — the one that costs nothing to operate and requires no tuning — is to position windows so the sun itself does the work.

Alexander, in *A Pattern Language*, comes close to this insight but never states it directly. Pattern 134 (Zen View) speaks of framing a single dramatic view; Pattern 159 (Light on Two Sides of Every Room) insists on multiple light sources. Neither addresses the *temporal* dimension — the fact that east light and west light are physiologically different, and that a room with both participates in the body's daily reset in a way that a room with one cannot. This pattern fills that gap.

The practical geometry is simple. A room with windows on two opposing walls — east and west — or on two adjacent walls — southeast and northwest — will receive both the morning signal and the evening signal. In Edmonton, at 53°N latitude, the summer sun rises north of east and sets north of west; in winter it rises south of east and sets south of west. A corner room with windows on both axes catches the full seasonal range. The deeper the window reveals, the more the light enters as a distinct event rather than a wash — a blade of sun across the floor in the morning, a warm glow on the ceiling at dusk.