Suppression of melatonin secretion in some blind patients by exposure to bright light.
Level 3 - non-randomized controlled study
Non-randomized controlled physiological study comparing blind patients and sighted controls.
PubMed 7990870 · doi:10.1056/NEJM199501053320102
What was done
Researchers measured the suppression of plasma melatonin concentrations during exposure to bright light in 11 patients with complete lack of conscious perception of light and 6 normal sighted controls. In two blind patients who showed light-induced melatonin suppression, testing was repeated with their eyes covered. Circadian entrainment and sleep history were also evaluated in all blind participants.
What was found
Exposure to bright light suppressed plasma melatonin by an average (±SD) of 69 ± 21% in 3 of the 11 blind patients, comparable to the 66 ± 15% reduction observed in normal controls. When two of these responsive blind patients had their eyes covered during light exposure, melatonin levels did not decrease. These 3 blind patients reported no sleep difficulties and maintained circadian entrainment. In the remaining blind patients, light exposure did not decrease melatonin in 7, and melatonin was undetectable in 1. All 8 unresponsive patients reported a history of insomnia, and 4 had demonstrated loss of circadian temperature rhythm entrainment.
Why it matters
This study demonstrated that non-visual ocular pathways can transmit photic signals to the hypothalamic circadian pacemaker in humans even in the complete absence of conscious vision, explaining why some blind individuals avoid circadian disruption.
Limits
The sample size was very small (11 blind patients and 6 controls). The specific anatomical and retinal mechanisms mediating the non-visual light pathway were not identified in the abstract, and temperature rhythm entrainment was only reported for a subset of participants.
Cited by
- supports Visually blind humans and animals can still photoentrain their circadian rhythms to light-dark cycles, whereas individuals who have had both eyes surgically removed lose light entrainment and free-run.