Caffeine to increase alertness can be OK for those who do not crash from the effort.

Sam,

You crash from effort. Caffeine will not overcome that crash. If an hour of computer causes a crash or headache, limit computer time to 45 minutes. Better yet, Learn the subtle signs that you are about to hit your limit. When you notice that sign (difficulty focusing, needing to reread a line of text, etc), stop and take a break. You may be fine to change to a different activity until you notice that sign of impending overload.