Ask for what directly? If he asked for stimulants that didn't interfere with sleep, he'd get replies suggesting... he look for ones with short half-lives like nicotine and avoid long-acting ones like modafinil. Short half-lives is the governing criterion, so he simply asked for it.
I think there are plenty of people that have tested various stimulants and know the effects those substances have on them, but who don't know the exact half-life of the substances.
I'm not even sure whether the real half life of caffeine is the thing that matters. When it comes to designing drugs there are quite a few things that are done in the delivery mechanism of the drug that can effect half-life.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.