How a temperature drop is linked to faster sleep onset
Core body temperature normally falls in the evening as part of the circadian wind-down, and this decline is associated with the transition into sleep. Research on oral glycine suggests it may promote peripheral vasodilation, increasing blood flow to the hands and feet so the body sheds heat and core temperature drops sooner. Faster heat loss is one proposed reason glycine has been linked to shorter time to fall asleep in small studies.
Glycine is also an inhibitory neurotransmitter in the brainstem and spinal cord, and it acts as a co-agonist at NMDA receptors, so its central effects are not limited to thermoregulation.
Strength of the human evidence
A handful of small Japanese trials reported that around 3 grams of glycine before bed improved subjective sleep quality and next-day alertness in people with mild sleep complaints. Sample sizes were small and effects modest, so glycine should be seen as a low-risk candidate rather than a proven treatment for any sleep disorder.
This is general educational information, not medical advice. Persistent insomnia warrants assessment by a qualified clinician rather than self-treatment.