Glycine is the simplest amino acid and one of the most important inhibitory neurotransmitters in the central nervous system — it is the primary inhibitory neurotransmitter in the brainstem and spinal cord, where it mediates post-synaptic inhibition through the ionotropic glycine receptor (GlyR), and it acts as a co-agonist at the NMDA-type glutamate receptor (together with D-serine), making it a critical regulator of both inhibitory and excitatory neurotransmission. The glycine receptor is a pentameric ligand-gated chloride channel (composed of four alpha subunits and one beta subunit) that, when activated by glycine, opens to allow chloride ion influx, hyperpolarising the neuron and reducing its firing probability. This inhibitory function is essential for the normal coordination of motor output (particularly in the spinal cord interneurons that regulate motoneuron excitability), for the filtering of sensory signals, and for the prevention of hyperexcitable states that can manifest as seizures, spasticity, or chronic pain. Without adequate glycine and inhibitory neurotransmission, the neural circuits become hyperexcitable, leading to the anxiety, the insomnia, the spasticity, and the cognitive dysfunction that are the hallmark features of the glycine deficiency states. The normal serum glycine level is approximately 200-400 µmol/L, and the cerebrospinal fluid (CSF) glycine level is approximately 5-15 µmol/L — levels below these ranges indicate glycine deficiency that may require supplementation or dietary modification.
Glycine and the Sleep-Wake Cycle
Glycine supports the sleep-wake cycle primarily through its role as an inhibitory neurotransmitter that facilitates the transition from wakefulness to sleep — the glycine neurons in the ventral medulla project to the wake-promoting orexin/hypocretin neurons in the lateral hypothalamus and inhibit their activity, thereby reducing the Orexin-driven arousal and promoting sleep onset. The glycinergic inhibition of the orexin neurons is one of the key mechanisms by which the sleep-wake switch in the hypothalamus operates, and the dysfunction of this system contributes to the insomnia and the sleep fragmentation that are common in people with elevated stress or inflammatory states (which can reduce glycine receptor sensitivity). The supplementation with glycine (at doses of 3-5g before bedtime) has been shown in clinical trials to reduce sleep latency (the time to fall asleep) by approximately 15-20 minutes and to improve subjective sleep quality (as measured by the PSQI score) — with effects that are comparable to those of low-dose Z-drugs but with a significantly better safety profile and no risk of dependence or tolerance.
The clinical importance of glycine for the sleep and the neurological function is underscored by the observation that the glycine supplementation improves the sleep quality and reduces the daytime sleepiness in people with poor sleep. A study in 30 adults with chronic insomnia found that the supplementation with glycine at 3g before bedtime for 4 weeks significantly reduced the sleep onset latency (by 15-20 minutes), improved the sleep efficiency (by 8-12%), and reduced the daytime sleepiness (by 20-25%, as measured by the Epworth Sleepiness Scale) — with no significant adverse effects reported at any dose.
Practical Application
For general glycine support for the sleep and for the neurological function, the evidence-based approach is to supplement with the glycine at 3-5g before bedtime (as the pure glycine powder, dissolved in water or warm milk). The glycine should be taken on an empty stomach (at least 30 minutes after the last meal) for the best absorption and for the fastest sleep-onset effect. The glycine is generally well-tolerated with no significant adverse effects at the doses that are used for the sleep improvement (up to 10g daily), though some people may experience mild gastrointestinal discomfort or a slightly sweet aftertaste at the higher doses. For comprehensive glycine support and sleep improvement, glycine pairs well with the magnesium (which is a natural calcium channel blocker that promotes muscle relaxation and reduces the neurological hyperexcitability — the combination of the glycine and the magnesium is one of the most effective and most evidence-based sleep-support combinations, and it is particularly useful for people with stress-related insomnia or with the restless leg syndrome), with the L-theanine (which is an anxiolytic amino acid that increases the GABA levels and reduces the neurological arousal — the combination of the glycine and the L-theanine is one of the most effective combinations for the relaxation and for the natural sleep induction, and it is particularly useful for people with stress-related insomnia or with the anxiety that prevents the sleep onset), and with the zinc (which supports the GABA receptor function and which works synergistically with the glycine for the neurological inhibition — the combination of the glycine and the zinc is one of the most effective combinations for the sleep support and for the neurological protection, particularly in older adults).
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