Whoop measures your heart rate variability. Oura tracks your sleep stages. Garmin counts your steps. Apple Watch knows your resting heart rate. Between them, they generate thousands of data points about your body every day.
None of them tell you whether your circadian clock is aligned.
That's a significant gap, because your circadian rhythm is the master system that governs when cortisol rises and falls, when your metabolism is most active, when your immune system peaks, when your cells repair themselves, and when you're cognitively sharpest. Everything these wearables measure is downstream of the circadian clock. They're measuring the outputs. Nobody's monitoring the input.
What the Circadian System Actually Controls
The circadian clock is not just a sleep-wake cycle. It's a 24-hour programme running in virtually every cell in your body — timed by the master clock in the suprachiasmatic nucleus of the brain, which synchronises itself to the external environment primarily through light.
When the clock is well-aligned: cortisol peaks in the morning to prime alertness, drops through the day, and is near-zero by bedtime. Insulin sensitivity is highest in the morning. Body temperature rises through the day, supporting physical and cognitive performance. Melatonin rises in the evening, preparing the body for sleep.
When the clock is misaligned — through shift work, jet lag, inconsistent sleep schedules, or chronic morning light deficiency — all of these rhythms shift or dampen. The knock-on effects include disrupted sleep, impaired glucose metabolism, reduced immune function, and increased risk of cardiovascular events. A 2023 meta-analysis estimated that habitual circadian disruption is associated with a 20–23% increase in type 2 diabetes risk.
The Light Signal Is the Input Nobody Tracks
Your circadian clock sets itself each morning using light. Specifically, melanopsin-activating 480–490nm light reaching the retinal ganglion cells in your eyes. The amount of this light you receive — and when you receive it — determines how well your clock is aligned to local time.
Most people in the UK in winter receive a fraction of the melanopsin stimulus their body clock needs to maintain alignment. Indoor work, dark commutes, overcast skies, late sunrises. The clock drifts. Slowly, gradually, you feel tired, flat, and slightly off-rhythm without being able to explain why.
No wearable currently quantifies this. Your daily light intake — expressed as a melanopsin-weighted photon dose — is invisible to every piece of health technology available today.
Why This Is About to Change
The Spectrumitor, part of the LightHealth range currently in development, is designed to track your daily light health score — a real-time estimate of your melanopsin dose relative to your personalised daily target, adjusted for your location and the season. Paired with the LightHealth app, it gives you the missing metric: not how you slept, but whether your body clock received the signal it needed to keep you aligned.
This is what Whoop, Oura, and Apple Watch can't tell you. It's also, arguably, the single most actionable data point available, because light is something you can directly control.
Explore the LightHealth technology.
LightHealth products are in development. Join the early access list and be first to know when we launch.
