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My Psychology text book says Serotonin causes "Sleep, arousal levels and emotion"

Does this really mean that when you are being aroused, Serotonin is released, which in turn makes you sleepy? If so, what evolutionary benefit would that have??

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This can be answered by reading the Wikipedia article on serotonin, which has a nice list of functions. – Armatus Mar 4 '13 at 18:09
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I've always wondered whether heightened serotonin was responsible for socially accepted phenomena such as post-coital tiredness and "morning glory"... – Polynomial Mar 4 '13 at 23:36
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You do realize that Arousal does not have anything to do with sex unless you expressly use the adjective Sexual to modify arousal From Medical Dictionary Arousal 1. a state of responsiveness to sensory stimulation or excitability. 2. the act or state of waking from or as if from sleep. 3. the act of stimulating to readiness or to action. – AMR Nov 27 '15 at 7:17
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According to this article [1], serotonin (5HT) inhibits sexual arousal:

Dopamine is generally facilitative to male sexual behavior; however, 5-HT is regarded as inhibitory. Antidepressants of the selective serotonin reuptake inhibitor class (SSRIs, including Prozac and Zoloft) impair ejaculatory/orgasmic function and frequently inhibit erectile function and sexual interest as well.

However, serotonin can also facilitate erections:

Systemic administration of the 5-HT1B receptor agonist anpirtoline impaired ejaculation in male rats. On the other hand, stimulation of 5-HT2C receptors with mCPP impaired ejaculation but facilitated erections in male monkeys, suggesting an increase in parasympathetic influence.

Serotonin release also causes sleepiness after ejaculation in males (source):

5-HT is released in the LHA at the time of ejaculation and promotes sexual quiescence during the postejaculatory interval

There are many signalling molecules used by the human body that have a different effect depending on the cell type they are detected by and the receptor type they activate.

I would also like to touch on your question about evolutionary benefit. Evolution does not care about you after you have reproduced. As long as an individual reproduces, evolution is "happy". For example, you think falling asleep after sex is counterproductive on the evolutionary scale? What about Mantises where the female will sometimes eat the male during or immediately after copulation. Falling asleep seems like a pretty small thing compared to that (source):

Although the praying mantis is known for its cannibalistic mating process in actuality it only occurs 5-31% of the time. Especially in laboratory conditions of bright lights and confinement, the female is more likely to eat the male as means of survival. "In nature, mating usually takes place under cover, so rather than leaning over the tank studying their every move, we left them alone and videotaped what happened. We were amazed at what we saw. Out of thirty matings, we didn't record one instance of cannibalism, and instead we saw an elaborate courtship display, with both sexes performing a ritual dance, stroking each other with their antennae before finally mating. It really was a lovely display". (7) There is one species, however, the Mantis religiosa, in which it is necessary that the head be removed for the mating to take effect properly. (5) Sexual cannibalism occurs most often if the female is hungry. But eating the head does causes the body to ejaculate faster. (3)

Anything that makes an individual less likely to reproduce will be selected against. Things that happen after reproduction do not affect the survival of the species and will not be subject to selective pressures.

References

  1. E. M. Hull, J. W. Muschamp, S. Sato, Physiology & Behavior 83 (2004) 291 – 307
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Arousal does not have anything to do with sex unless you expressly use the adjective Sexual to modify arousal From Medical Dictionary Arousal 1. a state of responsiveness to sensory stimulation or excitability. 2. the act or state of waking from or as if from sleep. 3. the act of stimulating to readiness or to action. – AMR Nov 27 '15 at 7:19
    
@AMR if you don't know what a word means, look it up in a regular dictionary, not a medical one. Here are a few: 1, 2, 3, 4, 5. Note that every one of them includes sexual arousal as the primary or secondary meaning of the word. In English, with no other context, arousal will always be taken to mean sexual arousal. – terdon Nov 27 '15 at 12:36
    
@AMR still, your rather aggressive comment urged me to check my guess and change my answer to be based on fact, rather than guessing. Happily, my guess wasn't far off the mark. Thanks. – terdon Nov 27 '15 at 12:37
    
It is a very small aspect of what neuroscience terms arousal. The use of the word in the context of the quote cited by the OP has nothing to do with arousal in the sense of sex and is used to describe activity during a wakeful state. You are sophisticated enough of a member to know the distinction and instead of pandering to the question you should have correctly described the use of the broader use of the term in a scientific sense and as it was likely used in the context of the textbook. – AMR Nov 27 '15 at 15:51

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