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??
There are many signalling molecules used by the human body that have a different effect depending on the cell type. My guess is that serotonin causes these different effects in different neurons. So, while serotonin is capable of promoting feelings of arousal and sleepiness, the two do not necessarily go together. If you look into this more deeply you will probably find that the different effects you describe are happening in different cells and at different times. Also, rather than arousal causing serotonin to be released, I think you will find that serotonin release causes arousal.
I would also like to touch on your question about evolutionary benefit. Although the action of serotonin is almost certainly not to induce arousal followed by sleepiness, I would like to point out that 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):
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.