Science Questions

Why does morning breath smell?

Sun, 12th Sep 2010

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nravinad (from twitter) asked:

Why is morning breath smelly, even after thorough brushing including the tongue before sleep?


Chris -   The reason for this is that when you go to sleep, your metabolic rate drops, your activity level drops, and your body secretions dry up. So your eyes produce fewer tears, other mucous membranes produce fewer secretions and your mouth produces less saliva. Your saliva has got two roles: one, it washes away bacteria and two, it also contains varous things like antibodies and other chemicals - lactoferrin and so on, that suppress the growth of microorganisms. So because at night you have less saliva, your mouth is dryer, and you've got fewer of these antimicrobial compounds like lysozyme, microbes tend to flourish more at night when we go to sleep.

The mouth contains a very diverse population of bacteria - there may be 80-100 different species in the mouth, more in some people. Some of those bacteria are capable of metabolising various sulphur containing compounds, and from those compounds they produce volatile chemicals that are quite whiffy, like hydrogen sulphide. So you wake up with a proliferation of these bugs in your mouth, more than you would have when you were making lots of saliva. As a result they make more of these whiffy chemicals and your breath smells, at least until you actually drink something to wash them away and begin making saliva that then helps to suppress their activity.


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