On November 3, 2025, the American Heart Association posted on its news site that long-term use of melatonin (a year or more) as a supplement for chronic insomnia may cause serious heart issues, as well as other health problems. The study has not yet been published, but the link is here.
The melatonin in your body is a hormone, which is secreted by the pineal gland, located in the middle of your brain, just under the pituitary gland. Melatonin, circulating in your bloodstream, settles you down to prepare for sleep as daylight wanes. The pineal gland secretes melatonin into the blood with the coming of darkness, which lasts through the night hours and decreases with the dawn as the sun rises. It’s most dense in your bloodstream around 2 to 3 a.m.
The recent AHA research is finding that people who experience SAD may have what is called subsensitivity to light in the retina, meaning that their eyes have less ability to take in and process light. Less light reaching the pineal gland would signal that darkness has fallen and it’s time to produce more melatonin. The hypothesis is that this untimely production of melatonin may explain the symptoms of SAD, such as depression.
This would explain why light therapy is so effective in relieving SAD. It adds back the light your eyes are not taking in. From the ClevelandClinic website (myclevelandclinic.org): “Melatonin helps your retinas become less responsive to light, which helps you wind down and feel less alert. When there’s more natural light the next morning, your retinas automatically become more active and your body stops releasing as much melatonin. melatonin, which could then affect your sleep and health.”