Glycine vs L-Theanine: Which Is Better for Sleep?

Mike HartnettEvidence-Based

7+ years researching longevity protocols. Runs a 20+ supplement daily stack. Built CoreStacks after losing his father to heart disease.

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Glycine vs L-Theanine at a Glance

What Is Glycine?

Glycine is the simplest amino acid your body makes. It’s also an inhibitory neurotransmitter, meaning it calms neural activity. But the reason glycine shows up in sleep conversations isn’t because of its neurotransmitter role — it’s because of what it does to body temperature.

Here’s the deal. Your core body temperature needs to drop about 1–2 degrees Fahrenheit for you to fall asleep. Glycine accelerates that drop by increasing blood flow to your extremities, pulling heat away from your core. A 2006 study in Sleep and Biological Rhythms showed that 3 g of glycine before bed improved subjective sleep quality and reduced daytime fatigue in participants with sleep complaints. The mechanism is elegant: you’re not sedating yourself, you’re triggering the same thermoregulation process your body uses naturally.

Beyond sleep, glycine has a longevity angle that doesn’t get enough attention. Some animal studies suggest glycine supplementation can mimic methionine restriction — one of the more strong life-extension interventions in rodent models. David Sinclair and Peter Attia have both referenced glycine’s role in longevity pathways, though the human evidence is still early. At 3 g per night for around $10/month, the risk-to-reward ratio is excellent. For the full rundown on sleep supplements, see our best sleep supplements guide.

What Is L-Theanine?

L-Theanine is an amino acid found almost exclusively in tea leaves. It crosses the blood-brain barrier and promotes alpha brain wave activity — the same brain state you hit during meditation or calm focus. This is why green tea feels different from coffee: the caffeine wakes you up while the L-Theanine keeps the jitters away.

For sleep, L-Theanine works by reducing anxiety and mental chatter rather than sedating you. It modulates GABA, serotonin, and dopamine activity, creating a calm mental state that makes it easier to transition into sleep. Andrew Huberman has included L-Theanine in his sleep stack, noting its ability to take the edge off without causing grogginess. He’s discussed it on the Huberman Lab podcast as part of his broader sleep optimization protocol — our Huberman sleep protocol breakdown covers his full approach.

The real advantage of L-Theanine is versatility. Take 100–200 mg in the morning with coffee for calm focus. Take 200–400 mg before bed for sleep. It does double duty in a way glycine simply doesn’t. If you’re dealing with both daytime anxiety and nighttime restlessness, one supplement handles both. For how L-Theanine compares against other calming supplements, check our ashwagandha vs L-Theanine comparison.

Key Differences Between Glycine and L-Theanine

Mechanism: Temperature vs. Brain Waves. This is the fundamental split. Glycine works on the body — it drops your core temperature to trigger sleep onset. L-Theanine works on the brain — it shifts your neural activity toward the relaxed alpha state. If your sleep problem is physical (you’re too hot, restless, can’t settle your body), glycine targets the issue more directly. If your sleep problem is mental (racing thoughts, anxiety, can’t shut your brain off), L-Theanine is the better match.

Versatility: Single-Use vs. All-Day. Glycine is a bedtime supplement. You take 3 g before sleep, and that’s its job. L-Theanine pulls double or triple duty. Morning focus, afternoon stress, nighttime wind-down — it works across contexts. If you’re trying to keep your supplement count low and your budget tight, L-Theanine gives you more for your money. Our longevity stack under $100 guide covers how to prioritize when budget matters.

Longevity Angle: Glycine Has the Edge. Glycine has something L-Theanine doesn’t: emerging longevity data. The methionine restriction mimicry pathway is genuinely interesting. A 2019 study in Aging Cell found glycine supplementation extended lifespan in rodent models. That’s not proof it’ll work in humans, but it’s a signal worth paying attention to. L-Theanine has some general neuroprotective data, but nothing as specific as glycine’s longevity connection.

Taste and Compliance. Glycine powder is actually sweet — it tastes good in water. Some people mix it into herbal tea before bed. L-Theanine in capsule form has no taste; as a powder it has a mild umami flavor. Neither is hard to take, but glycine’s pleasant sweetness is a genuine compliance advantage. You won’t dread your bedtime dose.

Can You Stack Glycine and L-Theanine?

Yes, and this is actually a solid combo. They work through completely different mechanisms, so there’s no redundancy. Glycine handles the thermoregulation piece while L-Theanine calms your mind. Together they cover both the physical and mental sides of sleep onset. Andrew Huberman has mentioned both in the context of sleep optimization, though not always as a combined stack. Combined monthly cost is about $20–35, which is reasonable for a non-pharmaceutical sleep protocol.

If you’re already taking magnesium for sleep (specifically magnesium glycinate or threonate), note that magnesium glycinate contains some glycine in its chelated form. Not enough to replace standalone glycine supplementation, but it’s contributing. Check our supplement interaction checker to map out how these combine with your existing stack.

What Experts Say

Andrew Huberman has discussed L-Theanine extensively on the Huberman Lab podcast as part of his sleep optimization toolkit. He’s noted its ability to promote relaxation without sedation, making it useful both as a sleep aid and a daytime calm-focus agent. His full supplement list is in our Huberman supplement stack breakdown.

Peter Attia has referenced glycine in conversations about longevity and metabolic health on The Drive podcast. He’s discussed the methionine restriction data and glycine’s role in collagen synthesis and connective tissue health. His broader longevity approach is covered in our Attia longevity protocol.

Dr. Matthew Walker, while not a supplement advocate generally, has discussed the importance of body temperature regulation for sleep onset — the exact mechanism glycine targets. His research aligns with glycine’s thermoregulatory approach to sleep improvement.

Brad Stanfield has reviewed L-Theanine’s evidence base on his YouTube channel, noting it has reasonable data for anxiety reduction and sleep quality improvement at 200–400 mg doses. He’s cautious about overselling any single sleep supplement but considers L-Theanine among the better-supported options.

Which Should You Choose?

Go with Glycine if you:

• Run hot at night or struggle with temperature-related sleep disruption
• Want a supplement with emerging longevity benefits beyond sleep
• Prefer powder form (glycine tastes sweet and mixes easily)
• Sleep problems are mostly about falling asleep, not daytime anxiety
• Want the cheapest effective option ($8–15/month)

Go with L-Theanine if you:

• Deal with both nighttime restlessness and daytime anxiety or stress
• Want one supplement that works for sleep, focus, and calm across the day
• Your sleep problems are driven by racing thoughts and mental overactivity
• You drink coffee and want something to smooth out the jitters
• You want the supplement backed by the most expert sleep protocols

My take: I use both, but if I had to pick one, it’d be L-Theanine for the versatility. I take 100 mg with my morning coffee and 200 mg before bed. On nights when I’m really struggling with sleep onset, I add 3 g of glycine. Together they run about $25/month, which is less than a single bottle of most sleep formulas. For a complete look at non-melatonin sleep options, see our melatonin vs apigenin comparison and our apigenin vs valerian breakdown.

Frequently Asked Questions

This article is for informational purposes only and does not constitute medical advice. Consult your healthcare provider before starting any supplement regimen.