# If Melatonin's half life is only 20 min (or up to 2 hours), why does drowsiness persist?

**URL:** <https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378>\
**Category:** Factual Questions\
**Created:** [January 20, 2011, 9:21pm UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378 "2011-01-20T21:21:23Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![miragesyzygy](https://avatars.discourse-cdn.com/v4/letter/m/46a35a/32.png) [@miragesyzygy](https://boards.straightdope.com/u/miragesyzygy)\
**Post date:** [January 20, 2011, 9:21pm UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/1 "2011-01-20T21:21:23Z")

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ok I’m no math genius here but from a chemistry formula I have to know,

Mass(after time t) = Mass(original) X (.5)^(t/(half life))

so if I took 1mg of melatonin 12 hours ago

m(in kg) = (10^-6)(.5)^(720/20)

=

10^-6 (.5)^36 = (10^-6)(1.5 X 10^-11)

or 1.5 X 10 ^-17 kg

or about 1.5 X 10^-11 mg

or basically nothing

if 2 hours  
10^-6 (.5)^6  
10^-6 (1.6 X 10^-2) or 1.6X 10^-8 kg or 1.6 X 10^-2 mg which is still very little.

Unless the amount of melatonin necessary to have any effect is smaller than the true amount which I expect to be somewhere between those 2 extremes?

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**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\
**Post date:** [January 20, 2011, 11:15pm UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/2 "2011-01-20T23:15:36Z")

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Melatonin doesn’t cause drowsiness directly. Its job is to trigger the onset of sleep. Once the trigger has been pulled its job is done.

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [January 21, 2011, 12:15am UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/3 "2011-01-21T00:15:52Z")

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Any complete answer will contain phrases like this:  
[Involvement of the nitric oxide cascade in melatonin-induced inhibition of long-term potentiation at hippocampal CA1 synapses.](http://www.ncbi.nlm.nih.gov/pubmed/20875465)  
The short and sweet of it is that melatonin acts to trigger other processes that operate on timescales longer than melatonin’s half-life.

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**Author:** ![Mahaloth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mahaloth/32/1079_2.png) [@Mahaloth](https://boards.straightdope.com/u/Mahaloth)\
**Post date:** [January 21, 2011, 1:50am UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/4 "2011-01-21T01:50:50Z")

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Wait. Melatonin actually does work to help people go to sleep? I thought the evidence was inconclusive.

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [January 21, 2011, 2:09am UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/5 "2011-01-21T02:09:11Z")

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> [@Mahaloth](#):
>
> I thought the evidence was inconclusive.

Last I looked for it, that was true. However, melatonin _does_ trigger some long lasting biochemical effects.

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**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\
**Post date:** [January 21, 2011, 2:39am UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/6 "2011-01-21T02:39:20Z")

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Internally generated melatonin is known to mediate the synchronisation of the circadian rhythm. How useful taking additional melatonin is in trying to modify the rhythm, or resynchronise it, is a different problem.

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**Author:** ![Rachellelogram](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rachellelogram/32/8689_2.png) [@Rachellelogram](https://boards.straightdope.com/u/Rachellelogram)\
**Post date:** [January 21, 2011, 2:48am UTC](https://boards.straightdope.com/t/if-melatonins-half-life-is-only-20-min-or-up-to-2-hours-why-does-drowsiness-persist/568378/7 "2011-01-21T02:48:15Z")

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Neurotransmitters can act as catalysts, too.