Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults

This is not a single experiment but a consensus recommendation document. The authors synthesised existing experimental evidence on how light intensity and spectrum (specifically the melanopic component, which activates the intrinsically photosensitive retinal ganglion cells or ipRGCs) affect:

**Circadian phase shifting** (how much the body clock moves earlier or later)

**Melatonin suppression** (the pineal gland's night-time hormone signal)

**Subjective and objective alertness** (self-reported sleepiness and EEG measures)

**Sleep onset latency** (time to fall asleep)

They then derived target ranges for three distinct periods:

**Daytime** (waking hours, ~08:00–18:00)

**Evening** (~18:00–22:00)

**Nighttime** (during sleep, including if waking in the dark)

The comparator is "typical indoor lighting" (often ~100–300 lux from warm-white LEDs or fluorescent tubes, which provide low melanopic EDI). The outcome measures are the physiological and behavioural responses listed above.

The recommendations are based on a meta-analysis of multiple experimental studies. The authors explicitly state the recommendations apply to **healthy adults aged 18–65 years** with no diagnosed sleep, circadian, or ophthalmological disorders. The underlying studies included a wide range of sample sizes (typically 12–48 participants per study), with both sexes represented, though most studies were conducted in laboratory settings with young adults (mean age ~25–35). The authors note that applicability to children, older adults (>65), and individuals with eye disease (e.g., cataracts, macular degeneration) or circadian disorders (e.g., delayed sleep phase syndrome) is not established.

The key metric is **melanopic equivalent daylight illuminance (melanopic EDI)**, measured in lux. This is a SI-compliant unit defined by the CIE (International Commission on Illumination) standard S 026:2018. It quantifies the light that activates melanopsin, the photopigment in ipRGCs. A standard lux meter cannot measure this; specialised spectroradiometers or calibrated melanopic EDI meters are required.

The underlying studies used:

**Polysomnography (PSG)** for sleep architecture and sleep onset latency

**Salivary or plasma melatonin** measured via radioimmunoassay or ELISA (for suppression and phase shift)

**Karolinska Sleepiness Scale (KSS)** for subjective alertness (1–9 scale, lower = more alert)

**EEG alpha/theta power** for objective alertness

**Dim-light melatonin onset (DLMO)** to assess circadian phase

**Actigraphy** (wrist-worn accelerometers) for sleep-wake patterns in some field studies

**Study design:** This is an expert consensus report, not a primary experiment. The authors conducted a systematic literature review of studies examining the effects of light on circadian, neuroendocrine, and alerting responses in humans. They then applied a formal consensus process (Delphi-like) among 10 international experts to derive the specific melanopic EDI targets.

**How the targets were derived:**

1. They identified the dose-response relationship between melanopic EDI and melatonin suppression (from published laboratory studies). The half-maximal suppression occurs at ~100 melanopic EDI (for a 1-hour exposure at night).

2. They identified the dose-response for circadian phase shifting: ~50–100 melanopic EDI for 1–2 hours can produce a ~30–60 minute phase shift.

3. They identified the threshold for alerting effects: ~200 melanopic EDI for 30–60 minutes increases subjective alertness by ~1–2 points on KSS.

4. They then set targets that would:

- **Daytime:** Provide sufficient light to maintain circadian entrainment and alertness (≥250 melanopic EDI at eye level for at least 2 hours)

- **Evening:** Minimise circadian disruption and melatonin suppression (≤50 melanopic EDI for the 4 hours before bedtime)

- **Nighttime:** Avoid any light exposure that could suppress melatonin or shift circadian phase (≤1 melanopic EDI during the sleep period)

**What this design can and cannot prove:**

**Can prove:** That the targets are consistent with the existing dose-response data from controlled laboratory experiments. The consensus process ensures expert agreement.

**Cannot prove:** That these specific targets are optimal for every individual, or that they will produce the predicted effects in real-world settings (e.g., with variable light exposure patterns, individual differences in sensitivity, or non-compliance). The recommendations are based on acute exposure studies (hours to days), not long-term (months to years) trials. There is no randomised controlled trial testing these exact targets against typical lighting.

**Major methodological weaknesses:**

The underlying studies are mostly short-term (1–3 nights) and laboratory-based, with artificial light sources. Real-world light exposure is dynamic and includes daylight.

Individual differences in melanopsin sensitivity (genetic polymorphisms in OPN4) are not accounted for.

The recommendations assume a "one-size-fits-all" approach, though the authors acknowledge that chronotype (morning vs. evening preference) may shift the optimal timing.

No blinding was possible in the original studies (participants could see the light), so placebo effects on subjective alertness cannot be ruled out.

**Daytime recommendations:**

**Target:** ≥250 melanopic EDI at the eye for at least 2 hours during the daytime (preferably in the morning, 08:00–12:00)

**Rationale:** At this level, melatonin suppression is ~70–80% complete (from a baseline of ~10–20 pg/mL to ~2–5 pg/mL), and subjective alertness is increased by ~2 points on KSS (from ~6 to ~4) compared to typical indoor lighting (~100 melanopic EDI)

**Secondary outcome:** Circadian phase advance of ~30–60 minutes when exposure occurs in the early morning (06:00–09:00)

**Evening recommendations:**

**Target:** ≤50 melanopic EDI for the 4 hours before bedtime (e.g., 18:00–22:00 for a 22:00 bedtime)

**Rationale:** At ≤50 melanopic EDI, melatonin suppression is <10% (from ~15 pg/mL to ~13 pg/mL), and circadian phase shifting is <15 minutes. At 100 melanopic EDI, suppression is ~30–40% and phase shift is ~30 minutes

**Secondary outcome:** Sleep onset latency is ~5–10 minutes shorter (from ~25 to ~15–20 minutes) when evening light is ≤50 vs. ≥200 melanopic EDI

**Nighttime recommendations:**

**Target:** ≤1 melanopic EDI during the sleep period (including if waking to use the bathroom)

**Rationale:** At 1 melanopic EDI, melatonin suppression is <5% (from ~20 pg/mL to ~19 pg/mL). At 5 melanopic EDI, suppression is ~15–20%. At 10 melanopic EDI, suppression is ~30–40%

**Secondary outcome:** Subjective sleep quality (PSQI) is not directly tested, but the authors infer that minimising nocturnal light exposure prevents circadian disruption, which is associated with poorer sleep quality in epidemiological studies

**Comparison to typical indoor lighting:**

A typical living room with warm-white LEDs at 200 lux provides ~50–80 melanopic EDI (evening-compliant if dimmed)

A typical office with cool-white fluorescent at 500 lux provides ~200–300 melanopic EDI (daytime-adequate)

A smartphone screen at 50% brightness at 30 cm provides ~10–20 melanopic EDI (evening-compliant if dimmed further)

A nightlight (e.g., 5 lux red light) provides ~0.5–1 melanopic EDI (nighttime-compliant)

To translate into plain English:

**Daytime bright light (≥250 melanopic EDI for 2 hours):** Equivalent to sitting near a window on an overcast day (not direct sun). This is about 2–3 times brighter than a typical office. The effect on alertness is roughly equivalent to drinking one cup of coffee (200 mg caffeine) — a ~2-point reduction on the 9-point sleepiness scale.

**Evening dim light (≤50 melanopic EDI for 4 hours):** Equivalent to a dimly lit room with a single 40-watt incandescent bulb (or a candle). This is about half the brightness of a typical living room. The effect on sleep onset is ~5–10 minutes faster — roughly equivalent to the effect of 5 mg of melatonin taken 30 minutes before bed.

**Nighttime darkness (≤1 melanopic EDI):** Equivalent to a very dim red nightlight or moonlight through curtains. This is about 1/100th of typical indoor lighting. The effect on melatonin is negligible — less than the suppression caused by a 30-second glance at a phone screen at full brightness.

**Acknowledged by authors:**

The recommendations are based on acute laboratory studies (hours to days), not long-term field trials (months to years)

Individual differences in sensitivity (age, genetics, prior light exposure) are not quantified

The targets assume a fixed 16:8 light-dark cycle (16 hours light, 8 hours dark), which may not suit shift workers or extreme chronotypes

The melanopic EDI metric does not account for the spectral composition of light beyond melanopsin activation (e.g., colour temperature, which also affects subjective comfort)

The recommendations are for healthy adults; applicability to children, older adults, and clinical populations is unknown

**Critical reader notes:**

**Industry funding:** The authors have no direct industry funding, but the CIE standard (S 026) was developed with input from lighting manufacturers (e.g., Philips, OSRAM). This could bias towards recommending specific light levels that are achievable with current LED technology.

**Publication bias:** The underlying studies likely overestimate effect sizes because negative results (no effect of light) are less likely to be published.

**Ecological validity:** Laboratory studies use fixed light exposure (e.g., 2 hours of constant 250 melanopic EDI), but real-world exposure is intermittent and variable. The effect of intermittent light is ~50–70% of continuous light at the same dose.

**No blinding:** Participants in the original studies could see the light, so placebo effects on subjective alertness and sleepiness are possible. Objective measures (melatonin, EEG) are less susceptible.

**Sample homogeneity:** Most studies used young adults (18–35), who have more robust circadian responses than older adults (>60), who show ~50% reduced melatonin suppression to the same light dose.

For someone running their own n=1 experiment:

### What to test

**Intervention:** Increase daytime melanopic EDI to ≥250 for at least 2 hours in the morning (08:00–10:00) AND reduce evening melanopic EDI to ≤50 for the 4 hours before bedtime.

**Dose:** Use a melanopic EDI meter (e.g., the "Light Meter" app on iPhone with the "Melanopic EDI" setting, or a dedicated device like the LYS or UPRtek meter). Alternatively, use proxy measures: daytime = sit within 1 metre of a south-facing window on a clear day (≥10,000 lux vertical illuminance = ~2,000–3,000 melanopic EDI) or use a 10,000 lux light box at 30 cm (~2,500 melanopic EDI). Evening = use dimmable warm-white LEDs (2700K) at ≤30 lux (~10–15 melanopic EDI) or a red light bulb (5 lux = ~1 melanopic EDI).

**Comparator:** Your usual lighting (e.g., typical office lighting in the morning, living room lights in the evening).

### Minimum meaningful duration

**Acute effects:** 3–7 days to see changes in sleep onset latency and subjective alertness (these respond within 1–2 days).

**Circadian effects:** 2–4 weeks to see stable changes in sleep timing and quality (circadian phase shifts take ~3–7 days to stabilise).

**Long-term effects:** 4–8 weeks to assess changes in mood, energy, and overall sleep quality (PSQI).

### What to measure (specific metrics)

**Primary outcome:** Sleep onset latency (time from lights-out to sleep, measured by actigraphy or sleep diary). A positive result is a reduction of ≥10 minutes (from ~25 to ~15 minutes).

**Secondary outcomes:**

- **Subjective alertness:** Karolinska Sleepiness Scale (KSS) at 10:00, 14:00, and 18:00 daily. A positive result is a reduction of ≥1 point (e.g., from 6 to 5).

- **Melatonin onset:** If you can afford salivary melatonin testing (e.g., from a lab like ZRT or DiagnosTechs), measure DLMO at baseline and after 2 weeks. A positive result is a phase advance of ≥30 minutes (earlier melatonin rise).

- **Sleep quality:** Pittsburgh Sleep Quality Index (PSQI) at baseline and after 4 weeks. A positive result is a reduction of ≥3 points (e.g., from 8 to 5; lower = better).

- **Daytime energy:** Single-item energy rating (1–10 scale) at 14:00 daily. A positive result is an increase of ≥1 point.

### Key confounds to control for

**Caffeine and alcohol:** Keep intake constant (e.g., same amount at same times each day). Caffeine blocks adenosine and can mask alertness effects. Alcohol disrupts sleep architecture and melatonin.

**Meal timing:** Eat dinner at least 3 hours before bedtime. Late meals increase core body temperature and delay sleep onset.

**Screen use:** If you must use screens in the evening, use "night mode" (blue light filter) at maximum setting and reduce brightness to ≤20%. Even with filters, screens at 50% brightness provide ~10–20 melanopic EDI, which is borderline for the evening target.

**Bedroom temperature:** Keep it cool (18–20°C). Heat disrupts sleep and can mimic the effects of light on sleep quality.

**Exercise timing:** Avoid vigorous exercise within 2 hours of bedtime (raises core temperature). Morning exercise may enhance circadian phase advance.

**Consistent bedtime/wake time:** Vary by no more than ±30 minutes. Circadian phase shifts require a stable sleep-wake schedule.

**Prior light exposure:** If you've been in dim light all day, your circadian system is more sensitive to evening light (dark adaptation). If you've been in bright light, you're less sensitive. Keep daytime light exposure consistent.

### What a positive result would look like

**Week 1:** You notice you feel more alert by 10:00 AM (KSS drops from 6 to 4). You fall asleep ~10 minutes faster (from 25 to 15 minutes).

**Week 2:** Your sleep onset is consistently ≤15 minutes. You wake up feeling refreshed (PSQI sleep quality item improves from "fairly bad" to "fairly good").

**Week 4:** Your PSQI total score drops from 8 (poor sleep) to 5 (good sleep). Your DLMO has shifted earlier by 30–60 minutes (you feel sleepy at 21:00 instead of 22:00).

**Week 8:** You maintain the improvements. You no longer need an alarm clock to wake up at your desired time. Your daytime energy is stable at 7–8/10 (vs. 4–5/10 at baseline).

**If