LIGHTING Daylight is not simply ‘bright’ – it
is spectrally broad, dynamically changing, directionally complex, and rich in longer wavelengths. It entrains circadian rhythms, supports retinal health, and influences essential mitochondrial activity. In winter months across the UK,
daylight exposure for healthcare workers may be negligible. Long shifts begin and end in darkness. Breaks are taken indoors. Patients in clinical environments may see little direct daylight at all.
If we remove natural light exposure and replace it with spectrally narrow electric light, what are the long-term consequences? We cannot yet quantify all outcomes.
Evidence links inappropriate light exposure to poor sleep and metabolic dysfunction.
However, increasing evidence links circadian disruption, poor sleep, metabolic dysfunction, inflammation, and ocular decline with inappropriate light exposure patterns. All-cause morbidity trends in modern
The mitochondrial question Mitochondria are the energy regulators of the cell. They produce ATP, regulate programmed cell death, influence inflammation, and play a role in metabolic and neurological function. Ageing is strongly associated with mitochondrial decline. Research led by Professor Glen Jeffrey at University
College London (UCL) has demonstrated that exposure to longer wavelengths in the red and near-infrared range can improve mitochondrial function in ageing retinal cells. Similar work by Professor Robert Fosbury and collaborators has examined the broader implications of spectral balance in modern lighting environments. Scott Zimmerman and Professor Alistair Nunn have contributed to the growing body of research around photobiomodulation and systemic mitochondrial support. The findings are not trivial. Improvements have been
observed in mitochondrial efficiency, cellular signalling, and markers associated with ageing and metabolic resilience.
Now consider this carefully: for most of human
evolution, daily exposure to natural daylight, which includes red and near-infrared wavelengths, was unavoidable. Today, many healthcare staff, patients, and residents spend the majority of their time indoors under lighting systems that are deficient in those wavelengths. We have changed the spectral diet of the population in less than two decades.
Introducing the concept of light nutrition We understand food nutrition. We understand vitamin deficiency. We regulate school meals and hospital catering accordingly.
But we do not yet regulate light nutrition.
society are multifactorial. Lighting is not the sole cause. But nor is it biologically neutral.
The indoor generation The average person in developed nations now spends over 90 per cent of their time indoors. Healthcare workers often exceed that. Electric lighting levels are typically far lower than outdoor daylight intensities. More importantly, they are spectrally constrained.
CIE guidance recommends consideration of melanopic equivalent daylight illuminance during daytime hours. This is progress. However, melanopic metrics address circadian entrainment, not mitochondrial stimulation via longer wavelengths. We are addressing one part of the spectrum while ignoring another. The IALD White Paper acknowledges that it is not possible to replicate natural daylight fully in electrically lit environments. The best we can do is avoid causing harm and improve spectral balance where feasible. That statement should give estates professionals pause.
Energy efficiency versus biological efficiency Healthcare estates operate under intense pressure: n Net-zero carbon targets n Budgetary constraints n Maintenance minimisation n Procurement frameworks prioritising lumens per watt.
We cannot recreate the sun indoors, but we can avoid designing environments that are biologically impoverished.
52 Health Estate Journal April 2026
The lumen, however, is weighted according to the Vλ curve, the sensitivity of the human visual system. Wavelengths outside that peak contribute less to measured efficacy. Near-infrared contributes almost nothing to lumens. In other words, the very wavelengths showing promise for mitochondrial support are penalised by current efficiency metrics. This creates a structural bias. Spectrally richer systems appear less efficient on paper, even if they may support human physiology more effectively. We would not design hospital catering solely around calorific efficiency. Yet we continue to design lighting solely around photopic efficiency. Is that sustainable in the long term?
AdobeStock / xiefei
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