Does glycine improve sleep?

Short answer

No study found longer sleep. Instead, in studies of 7 to 20 people who took about 3 g of glycine before bed, people woke up less at night and felt less tired the next day.

Does glycine improve sleep?
'A scoop of amino acid before bed → deep sleep' — does this formula hold?
Glycine is an amino acid your body also makes. It is sold as a supplement that is supposed to improve sleep when taken before bed. We checked what studies that gave it to people actually showed.
How many human studies are there?
Three studies in healthy people, all using 3 g
A 2024 review of glycine studies found 3 that looked at sleep in healthy people. All used 3 g taken 30 minutes to 1 hour before bed for 2 to 4 days, and sleep quality and tiredness improved. A 2026 review also found that all 3 glycine sleep studies improved at least one outcome.
The day after short sleep?
Less tiredness and better reaction test scores
7 people slept 25% less than usual for 3 days and took 3 g of glycine or a fake pill before bed. After glycine, they felt less tired the next day and scored better on a computer test of focused reaction. Sleepiness showed no clear difference.
Why would it affect sleep?
It pushed body temperature down
When rats were given glycine, blood flow to the skin rose and core body temperature dropped. In people, core temperature falls as we fall asleep. The researchers saw this temperature change as the way glycine works on sleep.
So does sleep get longer?
Fewer wake-ups, but total sleep stayed the same
13 people took 15 g of glycine-rich collagen (about 3.5 g of glycine) 1 hour before bed for 7 days. Night wake-ups dropped from 29.3 to 21.3. Total sleep time, time to fall asleep, and the share of time in bed spent asleep did not differ.
What about a month of use?
Fewer night wake-ups and faster sleep onset
20 people took 3 g of glucose 2 times a day for 4 weeks, then 3 g of glycine 2 times a day for the next 4 weeks. While on glycine, they woke up less often to urinate at night and took less time to fall asleep. There were no side effects.
Summary: glycine meant waking up less
It was not a pill that lengthens sleep
In studies using 3 g before bed, people woke up less at night and felt less tired the next day. No study found longer sleep. All studies had 7 to 20 people taking it for a few days up to 4 weeks, so about 3 g before bed, the amount the studies used, is the dose to go by.