Sleep & circadian bioeconomy

Sleep wearables are answerable to physics before physiology. The optical ones are photoplethysmographs: an LED shines into skin — green, because haemoglobin absorbs green strongly and ambient light interferes least there — and the detector records how much returns. Blood pulses change the absorbed fraction, so the waveform’s period is heart rate and its amplitude tracks local perfusion. Heart-rate variability read from the same signal reports autonomic balance, from which respiratory events and arousals leave measurable fingerprints: apnoeas cause cyclical desaturations and vasoconstriction that a well-placed sensor sees as oscillations in pulse and oxygen. Under-mattress mats and radar approaches skip contact entirely, reading ballistocardiography — the body’s mechanical recoil at each heartbeat.

The systematic weakness is staging. Polysomnography defines sleep by brainwaves; a wrist device sees movement and pulse, and infers sleep from stillness. That inference carries a known bias — a person lying awake motionless is scored asleep, so consumer devices systematically overestimate sleep duration and misjudge fragmented nights. The honest readout is trend, not night. Peripheral temperature adds a subtler channel: the distal skin of fingers warms in the evening as vessels dilate before sleep onset, and the distal-proximal gradient tracks circadian phase more faithfully than core temperature, which is flat across the day. That gradient, not the alarm clock, is what the body’s timing system actually consults.

The timing system itself is two clocks. The circadian pacemaker, the suprachiasmatic nucleus, is entrained daily by light through a dedicated route: melanopsin-containing retinal ganglion cells, most sensitive near 480 nm, report irradiance directly — which is why evening screen light delays the clock and why light-therapy glasses are a dosed intervention whose active ingredient is timing, wavelength and intensity, not merely brightness. Against the clock runs the homeostatic pressure of adenosine, accumulating in the basal forebrain across waking hours and cleared by sleep; caffeine works by occupying its receptors, which is why it shifts tiredness without paying back the debt. Healthy sleep is the alignment of the two clocks; misalignment — shift work, late light, travel — is measurable as hormone rhythms drifting apart from behaviour.

The regulatory frame follows the evidence bar. Sleep-apnea therapy devices and suspect-screening oximetry sit on cleared medical-device pathways; consumer trackers ride the general-wellness exemption, which asks less proof of the claim they display each morning. The measurement science is real either way — the same signals, staged with different rigour — and the open question is how much staging rigour a consumer format can carry at all. For where these signals go under clinical requirements, see wearable biodiagnostics; the breath channel some bedside devices add is the territory of breathomics.