A reader asks:

 Can light therapy alter endocrine hormone levels? There may be some evidence that TSH levels can be affected, but what about prolactin?


The strongest evidence for light therapy effects on hormones is for melatonin. When the lights are on and bright, nighttime melatonin production rapidly and reversibly turns off. There is certainly more to learn about this complex subject. In one study of SAD patients, light therapy did slightly lower plasma prolactin levels. However, there was no significant effect on other hormones studied. Another study showed no effect of light therapy on prolactin, but did find reduced TSH levels at night.