How does light therapy work to treat Seasonal Affective Disorder?



by Lydia


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Bright light therapy is widely prescribed as the first-line treatment of Seasonal Affective Disorder (SAD) and other depressive conditions. During these therapy sessions, the patient is exposed to artificial light that mimics natural sunlight.

Seasonal Affective Disorder is a form of depression signified by mood changes during the fall and winter months when there is less sunlight.

How Does Light Therapy Work To Treat Seasonal Affective Disorder (SAD)

Light therapy sessions consist of sitting or working near a device known as a light box or light therapy lamp. They produce a bright light that simulates daylight or sunlight to compensate for the lack of outdoor light during the darker winter months. Light boxes can be found in various designs, such as desk lamps or even wall-mounted fixtures.

Light therapy is believed to affect the brain chemicals linked to mood and sleep, easing SAD symptoms. People with different types of depression, sleep disorders, and other conditions can also use a light therapy box to alleviate and improve symptoms.

The first formal description of Seasonal Affective Disorder was created in the mid-1980s, according to the NCBI. The original study described a group of 29 patients who lived in a temperate climate and were experiencing depressive episodes characterized by hypersomnia, weight gain, and hyperphagia in the fall or winter. The patients were subjected to bright-white, full-spectrum fluorescent light of 2,500 LUX for 3 hours at dawn and 3 hours at dusk for two weeks.

The researchers noted a statistically significant antidepressant effect, as measured by the Hamilton Depression Rating Scale (HDRS), with effects generally observed within the first 3-7 days of treatment. Interestingly, when patients crossed over to a dim yellow light treatment of 100 LUX, there was a significant degree of relapse observed. This suggested that the antidepressant effect is specific to white light characteristics, such as illuminance and spectrum.

The Effect on Melatonin and Serotonin Production

Light therapy treatment may improve SAD symptoms by reducing your brain's production of melatonin and boosting its production of serotonin.

Melatonin is a hormone that plays a key role in the natural sleep-wake cycle. When it starts to get dark, your brain receives a signal to produce melatonin, which makes you feel sleepier. When light increases, your brain slows down melatonin production, signaling the body to wake up.

Serotonin is a hormone that aids in mood stabilization, which helps you feel happy. It impacts the whole body and enables the brain cells and other nervous system cells to communicate with each other. It also aids with sleeping, digestion, and eating. When you wake up in the morning, your brain should ramp up the production of this hormone, but if there is a lack of light, your brain may be slow to receive the usual environmental signals that it's daytime, resulting in lower levels of serotonin and moods.

Health Information To Consider When Looking For Light Therapy Boxes

Light boxes designed for light therapy should emit bright, strong light but no ultraviolet (UV) rays. Experts at the Mayo Clinic caution that some light boxes may not filter UV rays, which is something you should look out for.

The recommended light box is one with an intensity of 10,000 LUX positioned between 16 to 24 inches away from your face. With this type of light box, sessions may last between 20 to 40 minutes. If you're using a lower intensity device, such as one with 2,500 LUX, the sessions can go up to 2 hours.

The light from the light box must enter your eyes indirectly. Remember not to look at the light box directly, as the bright light can damage your eyes. Ensure to follow your doctor’s guidelines and the manufacturer’s directions. Light therapy requires consistency and should be followed under the strict guidelines from your doctor.

Sunrise alarm clocks, also known as dawn simulators, may also be useful for some people.

The Effect of Light Therapy on the Circadian Rhythm

When used consistently, exposure to this type of light can reset your circadian rhythm, helping control your daily sleep–wake cycle. Hence, those undergoing light therapy can fall asleep earlier at night or sleep in later, depending on their needs.

During a light therapy session, the retinal cells in your eyes recognize the light from the light box, which affects the production of melatonin and serotonin in your brain. The perception of light delays your brain’s melatonin production, thus waking you up and lifting your mood. Activating the hypothalamus at a certain time every day can restore a normal circadian rhythm, alleviating Seasonal Affective Disorder symptoms.

Light therapy is considered to be the most effective when practiced in the morning, as many people experiencing depression or circadian rhythm sleep disorders are considered to be “phase-delayed.” This means your internal body clock functions behind a normal circadian rhythm, and you likely sleep and wake up at later times than what is considered typical. Light therapy can also be useful for those who are “phase advanced” and feel tired early in the evening, like someone who works an overnight shift. For such individuals, afternoon or evening light therapy would be ideal.

Light therapy has also been shown to be effective as an accepted modality for treating Major Depressive Disorder, though it is underused.

Some people experience insomnia due to underlying circadian rhythm sleep disorder. These sleep disorders can develop naturally.

Jet lag tends to occur when you travel across time zones and your circadian rhythm remains in sync with your old location. Until your body catches up with your new area, you may feel sleepy in the middle of the day or find yourself waking up at odd hours of the night. Something as simple as spending some time outside can prove beneficial in dealing with jetlag, as it helps your biological clock realign to the sun. Light therapy can be a helpful treatment option when dealing with extreme jet lag after traveling across several time zones.  

Side Effects of Light Therapy

Mayo Clinic advises that light therapy tends to be safe for the most part. That said, you may experience some side effects, but they are usually mild and short-lasting. They may include:

  • Headaches
  • Nausea
  • Eyestrain
  • Irritability or agitation

These side effects should go away on their own within a few days of starting light therapy. You may also manage them by reducing the treatment time, increasing the distance between you and the light box, and taking breaks during long sessions.

Before you start therapy, make sure to talk to your doctor and follow up if any side effects become a challenge. If the treatment is right for you, it should help ease symptoms and deliver a higher quality of life.


1. Bright Light Therapy: Seasonal Affective Disorder and Beyond:

2. Seasonal Affective Disorder (SAD):

3. Light Therapy:,bright%20light%20therapy%20or%20phototherapy

4. Treatment - Seasonal Affective Disorder (SAD):

5. Melatonin:

6. What is Serotonin?:,sleeping%2C%20eating%2C%20and%20digestion.

7. Light Therapy for Insomnia Sufferers:

8. Bright Light Therapy for Nonseasonal Depression: An Emerging Intervention:


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