The 5 Best Supplements for Sleep (UK, 2024) — Ranked by Evidence

Poor sleep affects 1 in 3 UK adults. The supplement industry sells dozens of "sleep solutions" — but most are either underdosed, in the wrong form, or unsupported by clinical research. We ranked the five that actually have the evidence behind them.

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1. Magnesium Glycinate (300mg)

Why it works: Magnesium is a cofactor for over 300 enzymatic reactions, including GABA synthesis — the main calming neurotransmitter. Glycinate form specifically has the highest bioavailability and lowest GI side effects.

Evidence: A 2012 double-blind RCT in elderly subjects found 500mg daily improved sleep efficiency, sleep onset latency, and serum melatonin. Multiple systematic reviews confirm deficiency is associated with insomnia.

Optimal dose: 200–400mg elemental magnesium, 1 hour before bed.

Form matters: Avoid magnesium oxide — only 4% bioavailability. Glycinate or bisglycinate only.

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2. Ashwagandha KSM-66 (600mg)

Why it works: The main barrier to sleep isn't tiredness — it's cortisol. KSM-66 ashwagandha (standardised to 5% withanolides) reduces evening cortisol by up to 27% in clinical trials by modulating the HPA axis.

Evidence: A 2019 RCT found 600mg KSM-66 daily for 8 weeks significantly improved sleep quality, mental alertness on rising, and quality of life vs placebo.

Optimal dose: 300–600mg of a standardised extract (KSM-66 or Sensoril). Evening dosing.

Avoid: Cheap root powder with undefined withanolide content. Always look for the branded extract.

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3. L-Theanine (200mg)

Why it works: L-theanine promotes alpha brainwave activity — the relaxed but alert state — without sedation. It inhibits glutamate (excitatory) while promoting GABA (calming). Perfect for racing-mind insomnia.

Evidence: Multiple RCTs show 200mg L-theanine improves sleep quality, reduces sleep latency, and decreases pre-sleep anxiety. Suntheanine (the pure L-isomer) is the form used in research.

Optimal dose: 100–200mg. Can be taken alongside or stacked with magnesium glycinate.

Note: Not sedating on its own — works best for stress-related sleep issues.

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4. Glycine (3g)

Why it works: Glycine is an amino acid that lowers core body temperature — a key trigger for sleep onset. It also has GABA-mimetic effects and supports overnight muscle recovery.

Evidence: A 2012 study found 3g glycine before bed reduced fatigue, improved sleep satisfaction, and reduced daytime sleepiness the next day, even without changing total sleep time.

Optimal dose: 3g taken 30–60 minutes before bed. Can be added to water or a hot drink.

Bonus: Often cheaper per gram than other sleep supplements.

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5. Melatonin (0.5mg–5mg)

Why it works: Melatonin is the hormone that signals "it's night time" to your brain. Supplementing it can reset circadian rhythms, help with jet lag, and improve sleep onset when timing is the issue.

Evidence: Robust evidence for jet lag and shift work. More mixed for general insomnia — works best when your natural melatonin production is disrupted or your circadian rhythm is misaligned.

Important: Melatonin is available over-the-counter in the UK at low doses. Higher doses (above 0.5–1mg) don't necessarily work better and may cause grogginess.

Note: Start low (0.5mg), take 30–60 minutes before target bedtime. Not for long-term nightly use.

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What to avoid

- Valerian root: Mixed evidence; most RCTs fail to show significant benefits over placebo. - Lavender capsules: Weak evidence; mostly aromatherapy studies. - CBD: Insufficient human clinical data for sleep specifically. - Any product under-dosed: If a "sleep complex" contains 50mg magnesium oxide, it won't work.

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The optimal sleep stack

For most people with stress-related insomnia:

| Time | Supplement | Dose | |------|-----------|------| | Evening with dinner | Ashwagandha KSM-66 | 600mg | | 1 hour before bed | Magnesium Glycinate | 300mg | | 30 mins before bed | L-Theanine | 200mg | | 30 mins before bed | Glycine | 3g |

Key disclaimer: Supplements address nutrient-related sleep issues. Chronic insomnia may require CBT-I (cognitive behavioural therapy for insomnia), which has stronger long-term evidence than any supplement.