I think I'm sticking with sleep for now. I enjoy it, and I'm familiar with the literature a little better than other sub-fields. I may even try to volunteer in a sleep/circ lab the following semesters(!). In any case, I won't pretend to be an expert either... Definitely not, and I really don't want to step on anyone's toes. But sleep is FUN for me, so that's what I'm doing. For now. Unless I find another hobby. Undergraduates are allowed to do that, right?
ON TO THE PAPER!
We've got lined up for you one of the biggest, bestest papers in sleep research ever published - or at least I think so anyway - and it's pretty easy to find a .pdf even without a subscription to a journal service, because tons of courses have used this paper as part of their required readings.
Borbély, A., & Achermann, P. (1999). Sleep Homeostasis and Models of Sleep Regulation Journal of Biological Rhythms, 14 (6), 559-570 DOI: 10.1177/074873099129000894
Now, to make this clear, Borbély wasn't unveiling this model with this paper - he had worked on it long before, and this particular paper is among the long line of papers on the model. This one, though, is a well-consolidated version of the work.
So to make clear definitions: we're saying that sleep regulation is homeostatic in the sense that there is a baseline measure of some sort, and the organism exhibits a "pressure" to return to that baseline. What's being measured in the baseline in the case of sleep will soon be apparent.
This is what sleep looks like:
The top chart shows stages of sleep over time, from wake, to stages 1-4 and REM sleep. Keep in mind here, all this stuff has been common knowledge for a while - early on, sleep goes through the stages, from shallow to deep; most of the deep sleep is in the early part of the bout, whereas REM starts to happen in 90-120 minute epicycles (ultradian cycles) later in the bout.
The bottom chart is where the interesting stuff for Borbély is, mostly - the frequency of slow wave oscillations throughout the night. (If you've ever seen a sleep study, this is what they're looking at with the EEG-cap). Slow wave oscillations are strongest in the deeper stages of sleep (3 & 4), hence their name as "slow wave sleep" - REM lacks slow wave activity altogether.
This is again more evidence showing slow wave density - a spectral power density graph. The baseline (the perfectly straight horizontal line) on the left is the fourth cycle of sleep. Basically, what's going on here is showing that there's more slow wave activity earlier in sleep - inferable from the previous image - and that slow wave activity drops off and is less dense later in the sleep bout.
The right side shows averages across whole nights, with 40 hours of sleep deprivation and the following three bouts of sleep. The baseline is the third night, and the point here is just greater density in the night(s) immediately following the deprivation.
So for the homeostatic model - we see that deprivation drives up slow wave density. And further, we see that slow wave density is highest earlier in the night. So we have the suggestion that we can measure sleep "pressure" by the density of this slow wave activity. A few more figures cover this in detail (and I thoroughly encourage anyone interested to read the full paper), but basically longer sleep debt increases slow wave density - continued sleep drives it down.
Glancing over a lot of interesting research, but to get to the important point of it all, we have the point that sleep is homeostatically regulated. But there's another factor, and this one I'm not going to cover quite as thoroughly, because many before me have done it much better. Circacian processes are the cyclic processes with a period of about 24 hours, give or take a little bit of variation. Circadian processes are also important in sleep/wake, and part of Borbély's insight is working this well into the mix.
This, in essence, is Borbély's two-process model. Process S is the aforementioned sleep process - the tendency of slow wave activity to build up over the night, and drop off. There's a cool bit of math there in the drop-off during sleep (the hashed out area) - it's a logrithmic cliff (if I'm not mistaken), so similar in structure to, say, Newton's Law of Cooling for any physicists or chemists out there - the higher the starting point, the faster it drops. Process C is the circadian process, with the bar under the chart showing time of day in a 24 hour cycle. The distance between the S and C lines is the pressure for the organism to sleep - explaining, in a sort, why you're a little less tired mid-day after a marathon wake session, even going longer without sleep. [I just edited this to fix it - I mistakenly noted that the distance was indicative of slow wave density, which is actually JUST associated with Process S. Forgive my late night typing errors.]
This paper does throw out a couple of nice little springboards, too - Borbély leaves with a consideration of neurochemical factors, as well as a little chart suggesting that the process may be use-dependent rather than simply a factor of time. But I'll probably get to that another day. For now, enjoy Borbély's two stage model!