Does melatonin stop working if you take it every night?

The evidence doesn't show classic tolerance building up the way it does with sleeping pills — melatonin doesn't appear to lose its effect when taken nightly, but there's an important nuance about what it actually does.

The small, roughly 4–6 minute shortening in time to fall asleep seen in meta-analyses is the benchmark. A randomised placebo-controlled trial of prolonged-release melatonin taken nightly for 6 months in 791 adults with insomnia found that the effect was sustained over that period without signs of tolerance developing 3. The EMA's scientific review of prolonged-release melatonin found no significant melatonin-induced changes in lab parameters in studies lasting up to 18 months 3.

What is worth knowing:

• The effect was always modest — a few minutes off sleep onset, not a dramatic sedative hit. If it feels like it "stopped working," it may never have done much in the first place for chronic insomnia (one meta-analysis found no significant benefit in that group).
• The German insomnia guideline advises against long-term use, reflecting the fact that it addresses a symptom rather than the underlying cause.
• CBT-i (cognitive behavioural therapy for insomnia) has a much stronger evidence base for persistent sleep problems and doesn't carry the same long-term uncertainty.

There's no strong evidence that nightly use suppresses your own melatonin production 3, but the honest answer is that long-term data are still limited — well-controlled studies on extended use are scarce 3.