Light is the main signal your body clock runs on. Bright light in the evening delays and suppresses melatonin; that is well shown in the lab. Light boxes help seasonal depression, though the trials are small. What light does for hormones you can measure on a test is much less clear. Here is what holds up and what doesn’t.
Evidence: Mixed light therapy in depression · Strong evening light and melatonin, in lab studies · None test-guided use
The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for working out, in fruit flies, the genes that keep the body clock ticking on a roughly 24-hour cycle (Huang 2018). That is solid cell biology. Turning it into advice for people is where the evidence gets thinner, so this post grades each claim separately.
The grades in this post: Strong means good randomised trials agree. Mixed means the trials are few, small, short or unblinded, or disagree. Observational means studies show a link, not proof of cause. Mechanistic means the biology or animal work makes sense but it hasn’t been shown to change outcomes in people. None means there’s no usable evidence in people.
The master clock sits in a small part of the brain called the suprachiasmatic nucleus. It takes its timing mainly from light reaching special light-sensitive cells in the eye, which respond most to blue light (Hester 2021). Clocks in the liver, gut and other organs follow the master clock. That much is well established. How much a given change in your light habits shifts any of this in everyday life is less certain.
Cortisol rises sharply in the first 30–45 minutes after waking. This is the cortisol awakening response, or CAR (Stalder 2016; Stalder 2022). In healthy adults saliva cortisol climbs by roughly 50–160% in that window (Clow 2004). Light can enlarge it. In one small study, 12 people woken by a dawn-simulator lamp made more cortisol in the first 45 minutes than on normal mornings (Thorn 2004). That is a real effect, but it is a small study. Researchers still don’t agree whether a bigger or smaller CAR is better for health (Clow 2004).
I don’t use a stress test result to “prescribe” morning light. Getting outside in the morning is sensible for many reasons. It hasn’t been shown to fix a test number. Mixed One small study.
Evening light matters. In a lab study of 116 young adults, ordinary room light before bed delayed the start of melatonin in almost everyone and cut melatonin duration by about 90 minutes, compared with dim light (Gooley 2011). Reading on a light-emitting e-reader before bed reduced melatonin, delayed the body clock and made people take longer to fall asleep, compared with a printed book (Chang 2015). Strong Consistent lab studies.
Blue-blocking glasses are the obvious fix, but the trials don’t show much. A Cochrane review found the effect on sleep uncertain (Singh 2023), and a pooled analysis of three small trials found no significant change in measured sleep (Luna-Rangel 2025). An earlier, less strict review was more positive about falling asleep faster in people with sleep disorders or shift work (Hester 2021). Dimmer, warmer light in the evening is cheap and low-risk. Whether glasses add anything is unproven. Mixed Small trials that disagree.
One lab study found that healthy adults sleeping a single night with moderate room light on (100 lux) had higher next-morning insulin resistance than when they slept in a near-dark room (Mason 2022). One study, short term, but it points the same way as the evening-light work. Mixed One lab study.
For seasonal (winter) depression, light therapy beats sham in trials, but the trials are small and the overall quality of evidence is rated low (Chen 2024). Two small trials found light therapy and fluoxetine, an antidepressant, roughly equal; Cochrane rates that evidence low certainty (Nussbaumer-Streit 2021).
For non-seasonal depression the picture is similar: a mild-to-moderate benefit across 15–23 trials (Tao 2020; Tong 2024), with the evidence rated low quality (Tao 2020). The best single trial (122 adults, 8 weeks) found light alone, and light with fluoxetine, beat a sham device (Lam 2016).
NICE’s current depression guideline doesn’t recommend light therapy as such. It tells clinicians to advise people with winter depression that follows a seasonal pattern, who want to try light therapy instead of an antidepressant or a talking therapy, that the evidence for it is uncertain (NICE NG222, recommendation 1.4.39).
In bipolar disorder, light therapy has been tested in only six small trials, which together showed a benefit for depression (Bisdounis 2022). It should only be used under the care of the psychiatric team. Depression needs proper assessment. If you’re low, start with your GP. Mixed
Red and near-infrared light (photobiomodulation) is a different thing from body-clock light. I’ve graded it separately in Heat, Cold and Infrared: What the Evidence Shows.
Sleep timing, how long it takes you to fall asleep, morning energy and mood. These are what the trials measured. Melatonin metabolites and cortisol curves on urine or saliva tests have not been tested as a guide to light changes, so I don’t use them that way.
Your body clock runs on light. Bright days and dim evenings are the cheapest thing on this list. The evidence for that is better than the evidence for any gadget sold to fix it.
| Claim | Grade | Main source |
|---|---|---|
| Evening light delays and suppresses melatonin | Strong consistent lab studies | Gooley 2011; Chang 2015 |
| Light therapy for seasonal depression | Mixed positive, low-quality evidence | Chen 2024 |
| Light therapy for non-seasonal depression | Mixed positive, low-quality evidence | Tao 2020; Tong 2024; Lam 2016 |
| Light therapy as good as an antidepressant for seasonal depression | Mixed two small trials, low certainty | Nussbaumer-Streit 2021 |
| NICE recommends light therapy | None NICE says the evidence is uncertain | NICE NG222, 1.4.39 |
| Morning light enlarges the cortisol awakening response | Mixed one small study | Thorn 2004 |
| Blue-blocking glasses improve sleep | Mixed small trials that disagree | Singh 2023; Luna-Rangel 2025; Hester 2021 |
| Light during sleep raises insulin resistance | Mixed one lab study | Mason 2022 |
| Stress-hormone or melatonin tests can guide light changes | None untested | — |
I sell testing, consultations and personal training, so I declare an interest: the article’s grades apply to them too.
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