Light Therapy Side Effects

  • Duyen Nguyen MSci Human Biology, University of Birmingham


Are you feeling moody because of grey skies and long winter days? Do you long for clear days and sunlight? If so, light therapy may be what you need. As a non-invasive and increasingly popular alternative treatment, light therapy offers many health benefits – but at the same time, there is the risk of other side effects. 

What is light therapy?

Light therapy, also known as phototherapy, is a type of treatment which involves exposure to various wavelengths of light. There are 3 main types of light therapy (bright light therapy, blue light therapy, and red light therapy), which utilise different wavelengths of light and are used to treat different conditions. The type of light therapy you use will depend on the condition you are using it to treat. During treatment, patients are often required to sit within a certain distance and face in the direction of the light source so that the light can be properly received by the eyes.1 

Bright light therapy

Bright light therapy, as its name suggests, involves exposure to a bright light that mimics sunlight. This is often used to treat circadian rhythm disorders, such as delayed sleep phase syndrome (DSPS). It works by exposing the patient to bright light for a specific and regular length of time, which helps to adjust the patient’s biological clock to what is usually considered normal. Timing is critical for this to work, so the time at which the therapy is performed will depend on the patient’s condition.2 

Seasonal affective disorder (SAD) can also be treated using bright light therapy, simply by sitting by a light source for 30 minutes to an hour every morning. Research has shown that exposure to bright light at 10,000 lux for 30 minutes every morning results in an improvement in SAD.3 It has been suggested that SAD is triggered by reduced sunlight exposure, so bright light therapy can be used to simulate missing sunlight. As a result, it can help to regulate the production of serotonin, a hormone which can improve your mood, and melatonin, which affects energy levels. 

Blue light therapy

Blue light therapy is mainly used in dermatology to treat skin conditions due to its anti-inflammatory properties. It can be used to reduce acne by affecting oil production, as well as killing inflammatory bacteria and healing skin wounds. Additionally, it can help to improve mood and regulate sleep patterns. 

Photodynamic therapy (PDT) can also be used in combination with blue light therapy to treat cancer, which works by applying a certain type of drug to the skin and activating it with blue light. It is used to kill abnormal cells which could become cancerous due to damage from years of sun exposure. 

Red light therapy

Red light therapy utilises a particular wavelength of red or near-infrared light and is mainly used for skin conditions. It works by stimulating biochemical effects in cells, which affects the skin and internal organs.

Red light can help to reduce skin scarring and inflammation by boosting collagen production. Near-infrared light is absorbed more deeply by the skin, so it can affect the skin on a cellular level by affecting different genes involved in cellular functions.4 This can allow for relief of joint and muscle pain, and inflammation, due to its anti-inflammatory and analgesic properties which help to increase the rate of repair by affecting cellular healing mechanisms.

Types of light sources

Different types of light sources are used for light therapy, depending on the condition. Light boxes are the most common, as they are simple boxes which produce very bright light. The strength of the light produced will vary, with older models producing 2,500 to 5,000 lux, while newer models produce 10,000 lux. A higher lux will result in shorter light therapy sessions, with the length of the session decreasing in proportion to lux. Desk lamps behave similarly to light boxes, but are shaped differently so they may be easier for people to use. 

Light visors are light sources which are worn just like a cap so that they hang over the patient’s eyes. They are often used for hands-free activities, which allows for movement during sessions. They have the same effect as light boxes but may be more practical and easy to use. 

For patients with conditions such as SAD and sleep disorders, dawn simulators may work just as well as bright light therapy.5 They work to simulate a sunrise by gradually brightening a dark room over a set amount of time, which triggers the wake-up cycle of the body so that the patient wakes up naturally. As a result, this may improve wakefulness and help patients to adjust their circadian rhythms. Similarly, dawn simulators can also be used to mimic a sunset by dimming a bright room, which can help people with sleep disorders fall asleep at a normal time. 

Common side effects

Although light therapy has been very effective in both medical and holistic treatments, there are potential side effects to consider. These side effects can be positive or negative. It is rare for people to experience negative side effects.  However, for those who do, they are usually mild and don’t last long. 

Positive side effects

Light therapy can indirectly result in positive outcomes for your body and mental health. This often occurs as a result of stimulating the production of serotonin, a hormone associated with  higher energy levels and other positive mental effects. Positive side effects of light therapy may include:

  • Increased energy
  • Improvement in sleep quality, which includes longer nighttime sleep, a reduction in occurrences of waking up and fewer naps during the day6
  • Increased focus and productivity
  • Improvement in memory7
  • Reduced appetite and increased fat loss8
  • Decreased blood pressure: Whole-body blue light exposure has been discovered to reduce the blood pressure of participants, with similar effects as blood pressure-lowering drugs9  

Negative side effects

Due to the nature of light therapy, there may be negative side effects involved, especially during the early days of treatment. Headaches are one of the most common negative side effects of light therapy.10 Luckily, they tend to be mild and usually resolve after a few days. Eye or vision problems, including blurred vision and irritation of the eyes, are another common side effect. Both can be solved by temporarily increasing the distance to the light source used. 

Contrary to its use, light therapy can also result in sleeping problems, leading to tiredness and fatigue during the day. This is because light therapy suppresses the production of melatonin, which can result in insomnia if the timing is incorrect. However, the risks of this can be minimised by avoiding light therapy during the evening, and only using light therapy as and when directed by an appropriately trained healthcare professional. 

Some patients may also feel especially agitated or irritable, although this is not as common and is not regarded as serious. Moreover, some patients may feel overstimulated occasionally leading to hypomania, which could negatively impact their behaviour. As such, individuals who have bipolar disorder should use light therapy with great care and with the guidance of their healthcare professional. 

Although light therapy can be used to treat skin issues, there are potential risks as well. This is due to the increased exposure to UV radiation, which can worsen your condition and lead to adverse reactions. Light therapy can result in red and dry skin, although this may fade quickly. Minor pain, burns, and rashes have also been reported. Long-term treatment can increase wrinkles or may even result in the development of skin cancer later on in life. 

Factors affecting side effects

Of course, not everybody will be negatively affected by light therapy. The type and severity of symptoms experienced will vary depending on the patient, as well as the exact type of medication and light therapy used. Individuals who should be especially careful when using light therapy, or who should preferably use other alternatives, include those who: 

  • Have a past medical history of eye diseases, or diseases which involve the retina of the eye, such as diabetes
  • Have past medical history of mental health issues, including bipolar disorder
  • Are using photosensitising medications such as melatonin and lithium, which can result in adverse reactions to bright light
  • Are over the age of 65


Light therapy is a non-invasive and simple procedure that exposes patients to various wavelengths of light to affect different body parts. It has been shown to be very useful in treating patients medically and holistically, with uses ranging from dermatological problems to circadian rhythm disorders. Additionally, it has benefits such as an improvement in mood and sleeping patterns, an increase in energy, and a better ability to focus. 

Some people can experience negative side effects. However, the benefits of properly supervised light therapy tend to outweigh any negative side effects. Headaches and eye problems are the most common side effects, but they are generally of low severity and are short-lived. There may be long-term and more severe risks involved, but these are rare.


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  3. Campbell PD, Miller AM, Woesner ME. Bright light therapy: seasonal affective disorder and beyond. Einstein J Biol Med [Internet]. 2017 [cited 2024 Apr 26];32:E13–25. Available from: 
  4. Wunsch A, Matuschka K. A Controlled Trial to Determine the Efficacy of Red and Near-Infrared Light Treatment in Patient Satisfaction, Reduction of Fine Lines, Wrinkles, Skin Roughness, and Intradermal Collagen Density Increase. Photomed Laser Surg [Internet]. 2014 [cited 2024 Jan 24]; 32(2):93–100. Available from:
  5. Danilenko KV, Ivanova IA. Dawn simulation vs. bright light in seasonal affective disorder: Treatment effects and subjective preference. Journal of Affective Disorders [Internet]. 2015 [cited 2024 Jan 24]; 180:87–9. Available from:
  6. Rissling M, Liu L, Youngstedt SD, Trofimenko V, Natarajan L, Neikrug AB, et al. Preventing Sleep Disruption With Bright Light Therapy During Chemotherapy for Breast Cancer: A Phase II Randomized Controlled Trial. Frontiers in Neuroscience [Internet]. 2022 [cited 2024 Jan 24]; 16. Available from:
  7. Rojas JC, Bruchey AK, Gonzalez-Lima F. Low-level light therapy improves cortical metabolic capacity and memory retention. Journal of Alzheimer’s Disease [Internet]. 2012 Jan 1 [cited 2024 Apr 26];32(3):741–52. Available from: 
  8. Danilenko KV, Mustafina SV, Pechenkina EA. Bright Light for Weight Loss: Results of a Controlled Crossover Trial. Obesity Facts [Internet]. 2013 [cited 2024 Jan 24]; 6(1):28–38. Available from:
  9. Stern M, Broja M, Sansone R, Gröne M, Skene SS, Liebmann J, et al. Blue light exposure decreases systolic blood pressure, arterial stiffness, and improves endothelial function in humans. Eur J Prev Cardiolog [Internet]. 2018 [cited 2024 Jan 24]; 25(17):1875–83. Available from:
  10. Kogan AO, Guilford PM. Side Effects of Short-Term 10,000-Lux Light Therapy. AJP [Internet]. 1998 [cited 2024 Jan 24]; 155(2):293–4. Available from:
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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