Consider this natural law argument:
- Rational functioning is a basic good.
- One may never intentionally act against a basic good.
- In intending to fall asleep, one intends to stop rational function (i.e., thought).
- Therefore, it is wrong to intend to fall asleep.
It is tempting to dismiss the argument by saying that we need sleep to be rational. But that doesn't touch the argument. There are circumstances where the only way to survive is by killing an innocent person--but the end does not justify such a means. Likewise, if (1)-(3) are true, even if the only way we can maintain rational functioning is by sleeping, such a means is impermissible.
Aquinas discusses the question whether sex can be permissible in light of the fact that sex involves such an "excess of pleasure" that "it is incompatible with the act of understanding" (he attributes the latter claim to Aristotle). His answer is that sex can be done in accordance with reason, and what is done in accordance with reason is not sinful. He then says: "For it is not contrary to virtue, if the act of reason be sometimes interrupted for something that is done in accordance with reason, else it would be against virtue for a person to set himself to sleep." Unfortunately, Aquinas doesn't tell us which premise of the anti-sleep argument is false. It is not even clear that he has the same argument clearly in mind. In the case of sex, after all, the hampering of rational function looks like a side-effect (it's interesting that Aquinas doesn't just use Double Effect here) which need not be intended, while in sleep the lack of thought seems central.
For years I've struggled with the anti-sleep argument (but lost no sleep over it). I have two responses. Both of them leave (1) and (2) intact, but query (3). The first response is that in intending to fall asleep one intends to put off one's rational functioning rather than to stop it. A philosopher who leaves his office to walk around the beautiful campus intends that his rational functioning occur outdoors rather than in his office. Likewise, one might intend that one's rational functioning occur in the morning rather than late at night. And the reasons can be similar. The rational functioning outdoors or in the morning is likely to be fresher than in the office or late at night. The analogy here is strongest if one accepts a B-theory of time (and in fact, it may be an argument for a B-theory of time that it makes it easier to justify sleep).
The second answer is that sleep is not actually a cessation of rational function. It is very plausible that unconscious mental processes occur during asleep (it is clear that brain processes do!)--and an important part of sleep involves consciousness anyway. Sleep seems to be an important part of our rational functioning rather than an interruption. Clearly it is not an action against a basic good to switch from one kind of rational functioning to another, say turning one's mind from practical to abstract matters.
A difficulty with the second answer is that some people may not know that rational function continues in sleep. Yet surely such ignorance doesn't make it wrong to fall sleep. I agree. But we can also say that such a person may not intend the cessation of rational functioning. She may simply intend sleep, a particular natural human organic process. And if I am right that sleep is not constituted by a cessation of rational function, then we cannot even say that she "implicitly" acts against rationality or anything like that.
So, the anti-sleep argument fails, and natural lawyers can sleep with a sound conscience.