There are individual differences in metabolism rates of all drugs, so you might want to try some in the 3-5 hour range to see how they affect you. If tolerance worries you consider cycling some drugs or taking days off.
Nicotine could increase the clearance of caffeine as much as 50 %, you could use this to your advantage.
Consider raising your alertness nonpharmacologically, like exercise or cold showers for example. Consider working in an upright position.
In the other comment you said you have problems with sleep. There are several options to improve that side too, so that you might tolerate the stimulants better. This of course makes no sense if the problem doesn't lie in your waking hours. Melatonin you can probably get OTC. The nervousness induced by stimulants could also be dampened in several ways.
Thanks for your comment.
If I don't particularly like modafinil for its long half-life then I'll take a closer look at others. I'm sure my sleep doctor would be glad to give me a few samples of other stimulants.
I wasn't aware of the effect of nicotine on caffeine. Seems to be potentially useful.
Nonpharmacological treatments generally don't do much anything in my experience. Naps work fairly well, though they can leave me groggy. Exercise wakes me up during its duration, but not for much longer afterward (This is consistent with normal people's experiences)....
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.