BLT: A Treatment More Therapeutic Than The Sandwich

By Savanna Ledford

Figure 1: Standard Time Act of 1918 (Robinson, R. & Vick, A. (2019).

Does falling back an hour cause you to experience seasonal depression? You can thank Congress for that. Daylight savings time has been observed by most people in the United States since 1918 with the hopes that it will save energy consumption and improve the economy (Figure 1)2. However, many have expressed concerns towards adopting daylight savings time due to its effect on seasonal depression.

Seasonal depression, also known as seasonal affective disorder (SAD), is a type of mood disorder that occurs during a specific time of the year2. Most people impacted by SAD experience it during the fall and/or winter months, with symptoms lasting for about 40% of the year3. In fact, about 5% of adults in the United States experience SAD, 75% of which are women3,4. A study conducted in the United States showed that a mild version of SAD, known as the “winter blues”, affects about 10-20% of adults4. While SAD is more common in younger populations and women, you are at higher risk if you have a mood disorder, family history of mental health conditions, or live in high latitudes or cloudy regions4. Although researchers are not exactly sure what causes seasonal depression, there are five theories that explain how a lack of sunlight may cause the condition in those who are more prone to SAD4:

(1) biological clock change

(2) brain chemical imbalance

(3) vitamin D deficiency

(4) melatonin boost

(5) negative thoughts

First, the biological clock (circadian rhythm) theory, also known as the phase-shift hypothesis5 (Figure 2A), suggests limited exposure to light in the fall/winter months can cause difficulty in regulating mood, sleep, and hormones resulting in SAD symptoms5. These symptoms can include but are not limited to anxiety, sadness, irritability, extreme fatigue, inability to concentrate, and increased sleep4.

Second, the brain chemical imbalance theory focuses on the low serotonin activity in the brain. A lack of sunlight in the winter can cause serotonin levels to drop, leading to mood changes4. The serotonin hypothesis indicates that individuals with SAD have a greater binding potential at the pre-synaptic serotonin transporter5. Greater binding results in a greater uptake of serotonin into the presynaptic neuron (as indicated in Figure 2B)5. A greater uptake in serotonin decreases the available serotonin in the synaptic cleft which is associated with the outcome of depression symptoms5.

Figure 2: Proposed theories of SAD. Here we see (A) the phase-shift hypothesis showing a delay in the circadian rhythm with respect to time and (B) the serotonin hypothesis showing greater binding potential for SAD patients in the fall/winter months causing less available serotonin in the synaptic cleft. Campbell et al., (2019).

Third, serotonin is affected by vitamin D. Vitamin D helps boost serotonin levels in the brain4. Sunlight promotes vitamin D synthesis in the body, thus a lack of sunlight exposure during the winter may cause vitamin D deficits. Vitamin D deficits can then lead to low serotonin levels which impact mood4.

The fourth theory regards melatonin boost, which is what causes someone to feel more sluggish in the winter months4. Melatonin is synthesized in the pineal gland and is stimulated by β-adrenergic activity5. The synthesis of melatonin is inhibited when one is exposed to light4. Therefore, a lack of sunlight may cause an overstimulation of melatonin production, causing one to feel sleepier than usual4.

The fifth theory posits that those who experience anxiety, stress, and negative thoughts are prone to SAD4. This theory needs more research since scientists are unsure whether negative thoughts are the cause, or rather, an effect of SAD4. Since symptoms of SAD include feelings of hopelessness or worthlessness, investigators are interested in understanding if associated negative feelings cause SAD symptoms4. As we approach the long, northeast winter, it may feel at times that we will never see the sun again, putting us at risk for experiencing SAD symptoms due to the theories discussed. However, thanks to light therapy, we can now feel our best even on those gray, dreary days.

Bright light therapy (BLT) has been recognized as the first-line therapeutic modality for those with SAD6. Research into the utilization of BLT started in 1984 when a study showed that when exposing SAD patients to bright white light (increasing their daily light exposure), they experienced a reduction in symptoms7. Two meta-analyses that consisted of eight randomized, blinded, and controlled studies showed the efficacy of BLT6. Patients with SAD underwent BLT for four weeks and results showed that 50% of patients had a reduction in their symptoms6. It is hypothesized that BLT increases synaptic serotonin by reducing serotonin receptor binding which increases available serotonin, leading to an elevation in mood and the reduction of SAD symptoms9. In relation to melatonin, BLT inhibits β-adrenergic activity within the pineal gland, which plays a role in the inhibition of melatonin synthesis9.

The current strategy for optimizing BLT benefits is to undergo BLT in the morning8. Additionally, the light source should be a fluorescent light box with a diffusion screen and an intensity of 10,000 lux for 30 minutes8. Patients should position the box at a 30-degree angle from the line of gaze and at 12-24 inches away (Figure 3).

Figure 3: BLT set up (Healthline, 2021).

Now, before you run to Amazon or any store to purchase a light as seen in Figure 3, you should contact your medical provider first to ensure that what you are experiencing is SAD11. Patients using BLT have experienced side effects like headaches and nausea11, so it is important to work with your doctor. Additionally, keep in mind that the use of a light box device is not FDA-approved nor regulated11. If you and your doctor believe BLT is right for you, your doctor can help you make sure to buy a light box appropriate for SAD treatment11.

Besides BLT, there are other natural ways to reduce SAD symptoms, like increasing your dietary intake of vitamin D12, as low levels of vitamin D have been found in patients with SAD12. Keep in mind, the recommended dietary allowance for an adult is 600 international units (IU)13. Further research is still needed to explore if vitamin D supplements relieve SAD symptoms12, so it is again important to talk with a doctor before taking any vitamin supplement. Certain foods that can provide dietary vitamin D include salmon, egg yolks, and fortified foods14. Knowing the association of sunlight exposure with SAD symptoms exists, one can understand why scientists have expressed concerns towards adopting daylight savings time. Decreasing energy consumption and improving the economy are two important elements but, that should not cost citizens their health and well-being.


Seasonal affective disorder (SAD) affects 5% of adults in the United States and can last up to 40% of the year. Bright light therapy (BLT) reduces SAD symptoms and is considered among the first-line of treatment for patients.


  1. Robinson, R. & Vick, A. (2019). The Congress Project: The Standard Time Act of 1918. Retrieved from
  2. Healthline (2021). Daylight Saving Time: Falling Back Can Increase Seasonal Depression. Retrieved from
  3. American Psychiatric Association (2021). Seasonal Affective Disorder (SAD). Retrieved from
  4. Cleveland Clinic (2021). Seasonal Depression. Retrieved from
  5. Leersnyder et al. (2003).β1-adrenergic antagonists and melatonin reset the clock and restore sleep in a circadian disorder. Retrieved from
  6. Campbell et al. (2019). Bright Light Therapy: Seasonal Affective Disorder and Beyond. Retrieved from
  7. Rosenthal et al. (1984). Seasonal affective disorder. A description of the syndrome and preliminary findings with light therapy. Retrieved from
  8. Pail et al. (2011). Bright-light therapy in the treatment of mood disorders. Retrieved from
  9. Oldham, M. & Ciraulo, D (2017). Bright light therapy for depression: A review of its effects on chronobiology and the autonomic nervous system. Retrieved from
  10. Healthline (2021). 5 Best SAD LAMPS for Seasonal Affective Disorder and How to Use Them. Retrieved from
  11. Miller, M (2012). Seasonal affective disorder: bring on the light. Retrieved from
  12. NCCIH (2021). Seasonal Affective Disorder. Retrieved from
  13. Flo (2021). Too Much Vitamin D: Is it Possible?. Retrieved from
  14. NHS (2021). Vitamin D. Retrieved from

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s