Light therapy for PMS: does it work?

Light therapy for PMS has mixed evidence. Most of what's been tested is bright light exposure, using a light box well above normal room brightness, during the week or two before a period.

One small trial found bright light eased premenstrual low mood and tension more than a dim placebo light ( [3]). Another found morning, evening and placebo light all reduced depressive ratings from baseline by about the same amount ( [6]). But when researchers pooled the better-designed trials, the combined effect size was small, and its confidence interval crossed zero, meaning chance couldn't be ruled out ( [1]).

Most of this research was done in women with PMDD (back then called late luteal phase dysphoric disorder), not plain PMS, so results don't automatically transfer to milder premenstrual symptoms. If your main complaint before your period is low mood or depression rather than bloating or breast tenderness, you're closer to what these trials actually tested.

Here's how the main trials line up, from the strongest design to the smallest:

The pooled analysis of the better-designed trials found a weaker, less certain effect than the older, smaller crossover studies did. One trial that directly compared timing found morning light worked better than evening light ( [4]), though other trials found evening light effective too ( [3], [6]).

Trials that found a benefit used bright light daily during the luteal phase, the one to two weeks before a period, not just on a single day before a period.

Typical protocols looked like this:

• 30 minutes a day, starting roughly a week or two before the expected period ( [3], [6])
• Mood and depression scores checked again after about one week of nightly or morning use ( [4])
• One trial tested a single morning session and saw an immediate drop in negative mood that same day ( [2]), though this hasn't been shown to carry through a whole cycle

A full cycle or two of consistent, well-timed use is closer to what the research actually tested than a single session during a hard moment. Tracking your days on a DRSP chart can help you figure out when your own luteal phase starts.

None of the trials above reported serious harm from short daily light sessions. The side effects people commonly report with bright light therapy, in general, include:

• Eye strain or temporary blurred vision
• Headache
• Mild nausea
• Jitteriness or trouble sleeping if light is used too close to bedtime

These usually ease within the first few days as your body adjusts to the new light exposure and timing. Morning use, which most of the trials with positive results relied on, seems to cause fewer sleep-related side effects than evening use ( [4]).

If a headache or eye discomfort doesn't ease after about a week, or you notice any change in your vision, stop using the light box rather than pushing through it.

Bright light isn't the right tool for everyone. These are reasons to be cautious with light therapy:

• You have bipolar disorder or a history of mania (bright light can trigger a manic or hypomanic episode)
• You have an eye condition such as retinal disease, or take a medication that makes you more sensitive to light
• Bright light has triggered migraine with visual aura for you before
• You're already on a treatment for premenstrual symptoms and want to add light therapy on top of it

None of the trials reviewed here screened heavily for these conditions, so this caution comes from general light therapy safety guidance rather than the PMS studies themselves. If your premenstrual mood symptoms are severe enough that you're already working with a PMDD specialist, loop them in before adding light therapy to your routine.

Light therapy tends to work best alongside other support, not alone. These options also have evidence behind them:

• CBT: a systematic review of 32 studies found consistent, lasting improvement in PMS symptoms with cognitive behavioral therapy and psychoeducation, with no reported side effects ( [5])
• Aromatherapy: a meta-analysis of 8 trials found it lowered overall PMS scores, anxiety, depression and fatigue compared with no treatment, though the trials varied a lot in quality ( [7])
• Daily symptom tracking, so you and a doctor can see your actual pattern before changing or adding any premenstrual treatment

For a fuller rundown of what's backed by evidence, see PMS treatment: what actually helps. If your main symptom looks more like PMDD-level depression than general PMS, the evidence for light therapy is laid out separately in light therapy for PMDD.

A light box works over days, not in a single hard hour. Right now, something faster can help:

1. Step outside or sit by a bright window for a few minutes if you can. Natural daylight carries a similar cue even without a light box.
2. Name what you're feeling out loud or in writing, instead of trying to argue yourself out of it.
3. If a fight or a wave of tears catches you off guard, give yourself 20 minutes before you respond to anyone.

Niyora is a PMS and PMDD support app. Its Moon feature lets you write down what happened, see it from another angle, and draft a reply you can hold onto for 20 minutes instead of sending it straight away.

For more on what PMS actually is and why some days feel so different from others, see What is PMS?