Does Glycine Make You Tired? Understanding Its Effects on Sleep and Energy

December 24, 2025 •

2 min read

Over one-third of adults experience sleep problems at least once a week. As interest in natural sleep aids grows, many people wonder: does glycine make you tired? Surprisingly, clinical studies suggest that taking glycine can improve sleep quality, which in turn reduces next-day fatigue.

Quick Summary

Glycine can improve sleep quality by lowering core body temperature and acting as a calming neurotransmitter, which helps reduce next-day sleepiness and fatigue. Only excessive intake may cause drowsiness.

Key Points

Reduces Daytime Fatigue: Instead of causing tiredness, glycine supplementation has been shown to reduce daytime sleepiness and improve performance following restricted sleep.
Promotes Sleep Quality: Glycine improves subjective sleep satisfaction and sleep efficiency by shortening the time it takes to fall asleep.
Lowers Core Body Temperature: A key mechanism involves lowering the body's core temperature, a natural signal for sleep onset.
Acts as a Calming Neurotransmitter: Glycine calms the central nervous system, helping the mind relax and promoting a more restful state.
Minimal Side Effects: At commonly studied dosages, side effects are rare. Mild, temporary drowsiness is typically only a risk with very high intake.

The Paradox of Glycine and Sleep

At first glance, it may seem counterintuitive that an amino acid known to improve sleep wouldn't also cause drowsiness. Many conventional sleep aids are explicitly designed to induce a tired, sedative-like effect. However, glycine operates differently. Its benefits for sleep do not stem from a direct sedating action, but rather by supporting the body’s natural mechanisms for regulating sleep. The result is better quality rest at night, which translates to a more refreshed, clear-headed feeling during the day, not a tired one. The rare reports of drowsiness associated with glycine are typically linked to excessive dosages, not standard therapeutic amounts.

How Glycine Promotes Restorative Sleep

Instead of inducing a sedative effect, glycine works on multiple fronts to help prepare the body and brain for sleep, leading to deeper, more restorative rest. These mechanisms are crucial to understanding why it improves nighttime rest without causing daytime fatigue.

Thermoregulation: Lowering Core Body Temperature

A slight drop in core body temperature is a key biological signal that helps initiate sleep. Glycine has been shown to induce this hypothermic effect by increasing blood flow to the extremities, which helps the body dissipate heat. This physiological cooling effect is a natural precursor to sleep onset.

Neurotransmitter Modulation

Glycine acts as an inhibitory neurotransmitter in the central nervous system, helping to calm the nervous system and quiet brain activity. It modulates activity at NMDA receptors in the SCN, a circadian rhythm regulator. Glycine may also support natural sleep cycles by increasing serotonin, a precursor to melatonin, and potentially suppressing orexin neurons involved in wakefulness.

Improving Next-Day Alertness

Studies indicate that improved sleep quality from glycine supplementation leads to reduced next-day sleepiness and fatigue.

Glycine vs. Melatonin for Sleep

Glycine and melatonin are natural sleep aids with different mechanisms.


Feature	Glycine	Melatonin
Primary Mechanism	Lowers core body temperature and acts as a calming neurotransmitter.	Signals the body's circadian rhythm to initiate the sleep-wake cycle.
Mode of Action	Promotes relaxation and improves sleep quality naturally.	Directly signals the timing of sleep, potentially causing grogginess.
Effect on Next-Day Energy	Associated with reduced daytime fatigue and improved alertness.	Can cause grogginess in some people.
Primary Target	Supports thermoregulation and neurotransmitters.	Regulates the body's internal clock.

Dosage, Safety, and Potential Side Effects

Research on glycine for sleep has explored various approaches to supplementation. It is generally considered well-tolerated and safe for most people. Mild gastrointestinal discomfort is a rare side effect. Very high intake might cause drowsiness. Interactions with certain medications are possible. Consulting a healthcare provider is advised before starting any new supplement regimen.

Conclusion: The Final Word on Glycine and Tiredness

Glycine typically does not cause tiredness like sedative sleep aids. Instead, it improves sleep quality through natural processes, leading to reduced next-day fatigue and increased alertness. Supplementing with glycine can enhance sleep quality, making it a valuable tool for better rest.

Oral administration of glycine increases extracellular serotonin but not dopamine in the prefrontal cortex of rats

Frequently Asked Questions

Yes, commonly studied doses of glycine typically do not cause daytime drowsiness. Research has shown that taking glycine does not cause unwanted sleepiness, and in fact, can reduce daytime fatigue resulting from poor sleep.

Some individuals may feel benefits after the first dose, but research suggests noticeable improvements in sleep quality and reduced next-day fatigue are typically observed within one week of consistent use.

Glycine and melatonin work differently. Glycine promotes sleep by calming the nervous system and lowering body temperature, while melatonin signals the sleep-wake cycle. Your personal preference and specific sleep issues will determine which is more effective for you. Consulting a healthcare professional can help you decide.

Clinical studies exploring glycine for sleep have used various approaches to supplementation, often involving taking it before bedtime. It is advisable to follow product instructions or consult with a healthcare professional for guidance.

While glycine is found in protein-rich foods, achieving the levels used in clinical studies specifically for sleep effects can be challenging through diet alone. Supplements offer a more concentrated approach.

Glycine is generally considered safe for most people. Side effects are rare but can include mild gastrointestinal upset, particularly at high intake. Always consult a healthcare professional before starting any new supplement.

Glycine has been shown to be involved in the regulation of REM sleep by inhibiting motor neurons. This can help with conditions where individuals act out their dreams, known as REM sleep behavior disorder.