Several factors make the topic of your question a very broad issue. First, sexual alertness can be defined subjectively, as we perceive it, or physiologically, as a list of changes taking place in our body at different stages of alertness. Finding a correlation between subjective perceptions and physiological indicators is nothing but trivial and constitutes a field of research of its own. Second, men and women, or more broadly speaking males and females, are very different physiologically. These differences are manifested dramatically in the processes leading to sexual alertness, in the orgasm and later on. In light of this, studies in the field usually focus on only one of the sexes. In answering I will try to demonstrate the complexity of the issue and to illustrate (using anecdotes) the factors that are relevant to sexual alertness, including hormones. I will also relate to the drop in sexual desire after the orgasm.
Sketchings by French Illustrator Martin van Maele, depicting scenes of sexual alertness (1925).
The system responsible for sexual alertness in the human body serves two purposes. The goal of the first is sexual pleasure, which is the drive for mating. The second goal is reproduction itself. The body also contains a system for regulating the pleasure aspects, so that the motivations for achieving sexual pleasure do not interfere with the ability to reproduce. For instance, the sexual drive of women changes along the monthly cycle. It is assumed that there is a similar advantage for the acute drop in the sexual drive of men following orgasm.
The two main systems that function in the generation and perception of sexual alertness are the nervous system, which perceives the sexual stimulus and interprets it, and the endocrine system, which is responsible for hormonal regulation in the body. Hormonal regulation comes into play in many different stages, including the development of the reproductive system in the fetus and its maintenance throughout life. In the context of your question, the endocrine system has a role in regulating how the nervous system perceives or interprets signs of sexual stimulation. This is achieved by controlling the blood levels of a variety of hormones and peptides. For instance, the same stimulation (be it a smell, a sight or a touch) is interpreted in the brain in a very different manner before and after orgasm.
In a study from 2003 researchers measured the blood levels of various hormones during different stages of sexual stimulation, culminating in orgasm: most hormones did not show any fluctuations with time, but the levels of epinephrine and norepinephrine, two "general alertness" hormones, increased during orgasm and dropped right after it. Additionally, the study revealed that the blood level of prolactin also rises during orgasm, and stays up for a while after it. This observation, together with the fact that prolactin is known as an inhibitor of dopamine synapses in the brain, may imply that prolactin mediates the drop in sexual drive after the orgasm. However, aside from this rather circumstantial finding, no molecular mechanism relating prolactin to this phenomenon has been shown to date. Another problem in this context is the finding that a similar rise in prolactin takes place during the female orgasm. Subjectively, women are not known to lose their sexual drive after orgasm, and there are no known differences between men and women in the response to prolactin.
Two courting seagulls. Adopted from Wikipedia.
Despite also being present in women, testosterone is known as the principal male sex hormone. Studies have shown that when blood testosterone levels are artificially reduced, the male sexual drive drops. It should be mentioned that testosterone levels do not change during sexual stimulation, implying that this hormone regulates the basal sexual readiness prior to stimulation. In other words, testosterone affects the potential for sexual alertness once stimulation occurs.
In summary, the nervous system is responsible for perceiving and interpreting sexual stimuli. Despite the nervous system's dominant role in processing the stimuli and even in reaching orgasm, the endocrine system (hormones – you were right!) plays a major role in regulating the nervous system's interpretation of these stimuli. In this way, the pleasures of the physical stimuli prior to orgasm are replaced by an irritating tickle later on, and smells that are neutral become unpleasant. Sometimes, sensations that mask the stimulus may surface after orgasm: hunger or fatigue may be a result of the endocrine's system influence over the nervous system.
Some research questions in the field are still open, including identification of the molecular mediators (hormones and peptides) that participate in the endocrine system's regulation of the nervous system. Receptors for sex hormones have been found in the brain, but how these function in modulating the nervous system is still unknown.
If you find these explanations insufficiently clear or if you have further questions on this topic, please write about this in our forum, and we will relate to your comments. Your suggestions and constructive criticism are always welcome.