https://examine.com/supplements/melissa-officinalis/research/#nutrient-nutrient-interactions_valerian
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Lemon Balm and Valerian are commonly used in conjunction with one another for the purpose of sleep, as Lemon Balm is able to inhibit the GABA transaminase enzyme weakly (which converts GABA into Glutamate) while Valerian bioactives also weakly stimulates Glutamate Decarboxylase (which converts Glutamate into GABA); this combination is thought to cause a two-pronged stimulation of GABA concentrations in the brain secondary to inhibiting breakdown and stimulating synthesis of GABA.
The combination has been tested in a multicenter study where tolerability was excellent (no reported side effects) although the benefits also lacklustre (more self-reports of 'no effect' than 'benefit' in regards to sleep), although the amount of persons reporting better sleep (33%) was still greater than placebo (9%).
Study