Does the 'sleepy girl mocktail' actually work for sleep?

The "sleepy girl mocktail" — a viral bedtime drink pairing tart cherry juice with a magnesium supplement, usually topped with a splash of prebiotic soda — rests on two ingredients that each have some human trial data for sleep. But that data is thin, drawn from very small studies, and rated low in quality, so the drink is far from the reliable sleep aid the trend implies [s1][s2]. It is unlikely to hurt a healthy adult, and it may carry a genuine expectation effect; what it is not is a well-evidenced treatment.

That gap between confident marketing and modest evidence is the whole story with popular sleep supplements. The same applies to the other two names that come up most often, glycine and L-theanine: real but small studies, mostly in people who are not the insomnia patients the products are pitched to [s3][s4].

Tart cherry: one tiny trial

The case for tart cherry rests largely on a single pilot study. Researchers gave adults over 50 with insomnia 240 mL of Montmorency tart cherry juice twice a day for two weeks, in a placebo-controlled crossover design, and measured sleep with polysomnography [s1]. Only eight people completed it. In those eight, cherry juice increased sleep time by 84 minutes on polysomnography (P=0.0182) [s1]. The proposed mechanism is that a compound in the juice slows the breakdown of tryptophan, the precursor to sleep-regulating serotonin and melatonin [s1]. An 84-minute gain sounds dramatic, but it comes from eight people in a pilot explicitly designed to generate hypotheses, not to settle them — the authors present it as preliminary [s1]. Tart cherries do contain small amounts of melatonin, which is the more direct sleep signal; for melatonin taken on its own, see the melatonin evidence.

Magnesium: cheap, popular, weakly supported

Magnesium is the mocktail's other active ingredient and one of the most-bought sleep supplements. A 2021 systematic review pooled the randomised evidence and found only three trials, in 151 older adults across three countries [s2]. Pooled, magnesium shortened the time to fall asleep by 17.36 minutes versus placebo (95% CI −27.27 to −7.44, P=0.0006), while total sleep time improved by 16.06 minutes but not significantly [s2]. The reviewers judged the trials at moderate-to-high risk of bias and the evidence low to very low quality, concluding the literature is too weak for physicians to make firm recommendations — while noting magnesium is cheap and low-risk [s2]. In other words, a possible small benefit resting on shaky studies. For magnesium beyond sleep, see what the evidence shows on magnesium supplements and a trial of the magnesium L-threonate form.

Glycine and L-theanine: small studies, wrong patients

Two other supplements dominate the sleep aisle. Glycine, an amino acid, has been tested at 3 grams before bed: in a study of sleep-restricted healthy volunteers, it reduced next-day fatigue on a visual-analogue scale and improved performance on a psychomotor vigilance test, though it did not significantly change plasma melatonin [s3]. Notably, that work came from researchers at a glycine manufacturer, a provenance worth weighing [s3]. L-theanine, from green tea, was tested at 200 mg a day for four weeks in a randomised crossover trial of 30 healthy adults: Pittsburgh Sleep Quality Index scores fell (P=0.013), with improvements in the sleep-latency and sleep-disturbance subscales [s4]. Both are small studies, largely in people without a diagnosed sleep disorder, measuring subjective outcomes over short windows [s3][s4].

What it means

None of these supplements has the evidence base of behavioural treatment for chronic insomnia, and the trials that exist are small, short and mostly low quality [s1][s2][s3][s4]. The sleepy girl mocktail is best understood as a pleasant, low-risk bedtime ritual whose main ingredients carry weak signals rather than proven effects — and a consistent wind-down routine may itself help. Supplements are not risk-free for everyone: magnesium can cause diarrhoea in larger doses and interacts with some medicines, so they are worth checking against your own circumstances [s2]. For other viral sleep aids rated against the evidence, see the sleepmaxxing hacks, weighted blankets and valerian. None of this is medical advice.

Sources

• Pilot Study of the Tart Cherry Juice for the Treatment of Insomnia and Investigation of Mechanisms — American Journal of Therapeutics, 2017-09-11
• Oral magnesium supplementation for insomnia in older adults: a Systematic Review & Meta-Analysis — BMC Complementary Medicine and Therapies, 2021-04-17
• The Effects of Glycine on Subjective Daytime Performance in Partially Sleep-Restricted Healthy Volunteers — Frontiers in Neurology, 2012-04-18
• Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial — Nutrients, 2019-10-04