Here are three types of conscious experience, or "phenomenology", that it's difficult to deny:
(1.) sensory experience (like the experience of redness produced by looking at a red object, like the taste of saltiness in one's mouth),
(2.) imagery experience (like a picture in one's mind's eye of the Taj Mahal, or a tune or sentence running silently through one's head), and
(3.) emotional experience (the rush of anger, the shock of sudden fear).
Some scholars think one or more of these reduces to or is a variant of another (maybe emotional experience is just sensory experience of the body [as William James says], maybe imagery experience is just a faint version of sensory experience [as David Hume says]). But clearly we have all three types of experience.
It's sometimes argued that we also have other types of experience, but there has never been a consensus on what the other types are. Imageless thought or "cognitive phenomenology" is one suggestion, which has been getting a lot of attention recently (e.g., by Charles Siewert, David Pitt, and Russ Hurlburt) -- the supposed experience we have of thinking something which is not just a matter of having images or emotional experiences of a certain sort, but which has its own irreducible phenomenology. Early in the 20th century, E.B. Titchener argued against the existence of such cognitive phenomenology, suggesting that it mostly reduces to visual images, inner speech (both forms of imagery), and the like. More recently, William Robinson and Jesse Prinz have argued similarly against it.
How about the experience of feeling sleepy? I can't recall any good discussions of this in the philosophical or psychological literature. (If I've missed something, please let me know!) Is that reducible to one or more of those three types of experience?
Maybe it's a type of sensory experience? To think clearly about this, we need first to think about what other kinds of experiences are sensory -- for the categories above are clearly incomplete unless we have a fairly broad notion of "sensory", such that pains count as sensory experiences and feelings of muscular tension and limb position and feelings of fullness or discomfort in the alimentary canal. Is feeling sleepy sensory in the same way these other experiences are -- a matter of experiencing how things are going in your body?
As it happens, I'm sleepy right now. (Hence the inspiration for this post.) This slight headache, this feeling that I'm tempted to describe as a heaviness near my eyes -- those seem like sensory experiences. But there's more to sleepiness than that. A lassitude in my limbs? Is that sensory? But could I have this very same heaviness and lassitude and not feel sleepy? Or feel sleepy without this heaviness and lassitude? My guess is -- but it's only a guess -- that there's something more.
Also: Sleepiness is as much a state one one's brain as of one's body. I can understand how detecting the condition of one's body is, in an appropriately broad sense, sensory; but is detecting the condition of one's brain also sensory? That doesn't seem right. The brain does all kinds of self-monitoring and engages in all kinds of feedback loops; would those, too, be "sensory"?
So maybe sleepiness is, experientially, an emotion? It has a valence, like emotion (negative, usually), and perhaps a typical facial posture. But it doesn't appear on most psychologists' lists of emotional states (sadness, happiness, fear, anger, surprise...). It doesn't seem to arise, usually, as a reaction to how things are going for you and those you care about, for example in response to a change for better or worse in one's condition, as emotions typically do. But maybe not all emotions are like that? (Is surprise even like that?) What is an emotion, exactly? Well, we won't solve that question today.
Or is the experience of sleepiness sui generis, just its own unique sort of thing? And if so, then the feeling of being well-rested, too? And who knows what all else? Feeling energetic? Competent? Lusty? Healthy? The boxes in which we're supposed to fit things, the categories of experience -- their borders seem no longer clear, or they won't stand still....