The Science Behind the Healthy Home

Most homes are lit for one job: helping us see. A single ceiling fitting, often cool and bright, does the work from dawn until bedtime. But light does more than illuminate a room. It is also the most powerful time signal the body receives, and the light we live under in the evening tells us, quite literally, what time it is. Here is what the research says about light at home, and how to put it to work room by room.

Light is a signal, not just illumination

Alongside the rods and cones we use to see, the eye contains a third type of light-sensitive cell. These cells don't help us see at all. They report to the body's internal clock, the circadian system that times sleep, alertness and the release of melatonin, the hormone that prepares us for sleep.

Ordinary evening light is enough to register. In a widely cited study published in the Journal of Clinical Endocrinology & Metabolism, researchers compared adults spending the hours before bed in typical room light (under 200 lux) with the same adults in dim light (under 3 lux). Under room light, melatonin onset was delayed in 99% of participants, and the overall duration of melatonin release was shortened by around 90 minutes.¹ The study was carried out in a laboratory with healthy young adults, so it isn't a perfect model of every home. Its message is still clear: the brightness of a room in the evening matters.

Why colour temperature matters

Colour temperature describes the warmth of a light and is measured in Kelvin (K). Candlelight sits around 1800–2000K, a classic incandescent bulb around 2700K, and cool daylight-style LEDs at 5000–6500K.

Warmth matters as well as brightness. In a study published in PLoS ONE, participants spent two evening hours under light of the same brightness at different colour temperatures. The cool 6500K light suppressed melatonin significantly more than warm light at 2500K or 3000K, and also left participants feeling more alert.² That is useful at 10am, and less so at 10pm.

That is the thinking behind dim-to-warm. As you dim a Tala dim-to-warm bulb, its colour temperature falls with the brightness, just as firelight or an old incandescent bulb would. The Sphere IV E27 Dim-to-Warm LED Bulb moves from 2800K at full brightness down to 2000K, so the same lamp can give clear light for the afternoon and a low, amber glow after dark.

Lower, softer, dimmable

Brightness and warmth work together, and the easiest way to control both is to stop relying on the big light. Lighting designers build rooms in layers: ambient light to open up a space, task light for reading and cooking, and accent light for atmosphere. In the evening, the most comfortable light tends to sit lower, closer to eye level, and on a dimmer.

A wall dimmer such as the Double Rotary Wall Dimmer in Brass hands that control back to you, and paired with dim-to-warm bulbs, a single turn changes both the brightness and the warmth of a room. Where rewiring isn't an option, portable lamps do the same job anywhere. The Muse 2.0 dims from 2800K down to 1800K, while the Mantle Portable Lamp gives a steady 2200K glow, and both run cordless.

Screens before bed

The light we bring into bed with us counts too. In a study published in PNAS, adults read for four hours before bed on a light-emitting tablet on some nights and from a printed book on others. After reading on the tablet, participants took longer to fall asleep, released less melatonin in the evening, showed later timing of their internal clock and felt less alert the next morning.³

The simplest response is the one sleep researchers return to again and again: keep screens out of the bedroom, and read from paper under a warm, low lamp instead. A bedside or reading lamp such as the Heath Table Lamp, available with a dim-to-warm bulb and fitted with an in-line dimmer, gives enough light for the page without flooding the room.

Waking with light

Mornings are the other half of the picture. Light that gradually brightens before you wake, often called dawn simulation, has been studied as a gentler alternative to an alarm alone. In one study, thirty minutes of artificial dawn before waking reduced sleep inertia (that groggy, slow-to-start feeling) and increased how alert participants felt, compared with light switched on at the moment of waking.⁴ A separate study found that a dawn-simulating light significantly reduced complaints of sleep inertia.⁵ These are small studies focused on how people feel on waking rather than on sleep itself, but the direction is consistent.

The Wake Sleep Light, designed with Thomas Heatherwick Studio, is built around this idea. It begins the day with a gradual rise of light and sound and ends it with a warm glow, dimming from 3100K down to 1800K, with sleep sounds to wind down to.

Flicker and colour: the details you can't see

Two qualities of light rarely appear on the box, yet affect how comfortable a room feels.

The first is flicker. Many light sources pulse rapidly, often too fast to notice. In a well-known study of office workers published in Lighting Research and Technology, the incidence of headaches and eyestrain more than halved when older fluorescent lighting was replaced with high-frequency lighting that removed most of the flicker.⁶ That research looked at fluorescent tubes rather than LEDs, but LEDs can flicker too, depending on how they are driven. The Tala bulbs and lamps featured here are all specified at below 5% flicker.

The second is colour rendering, measured by the colour rendering index (CRI). CRI scores how faithfully a light shows colour compared with daylight, which scores 100. Many household LEDs sit around 80; the Tala products featured here are rated 95 or above, so wood, fabric, skin tones and the pages of a book look as they should.

Putting it into practice, room by room

• Living room: layer light at three heights, with a pendant above, floor and table lamps at eye level and low accent light, and save the brightest setting for daytime.
• Kitchen: keep downlights for cooking, and add a warm, dimmable layer over the table or island for everything that happens after.
• Bathroom: bright for the morning routine, then low and warm before bed. Choose an IP-rated fitting such as the Firth Wall Light (IP65, 2000–3000K dim-to-warm).
• Bedroom: screens out, a warm lamp for reading, and gradually brightening light in the morning.
• Everywhere: start with the bulb. Warm colour temperature, high colour rendering and low flicker improve every fitting you already own.

For more ideas on how light can shape each room, explore the healthy home lighting guide.

Sources

1. Gooley JJ, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. J Clin Endocrinol Metab. 2011;96(3):E463–E472. doi:10.1210/jc.2010-2098
2. Chellappa SL, et al. Non-visual effects of light on melatonin, alertness and cognitive performance: can blue-enriched light keep us alert? PLoS ONE. 2011;6(1):e16429. doi:10.1371/journal.pone.0016429
3. Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS. 2015;112(4):1232–1237. PubMed
4. Van de Werken M, et al. Effects of artificial dawn on sleep inertia, skin temperature, and the awakening cortisol response. J Sleep Res. 2010. Full text
5. Gimenez MC, et al. Effects of artificial dawn on subjective ratings of sleep inertia and dim light melatonin onset. Chronobiol Int. 2010;27(6):1219–1241. PubMed
6. Wilkins AJ, Nimmo-Smith I, Slater AI, Bedocs L. Fluorescent lighting, headaches and eyestrain. Lighting Research and Technology. 1989;21(1):11–18.