Stimulants are the classic answer here (caffein b vitamins). But i don't think that there is anything that acts a a proper substitute for sleep. your best bet is to restructure your schedule so you can get more sleep rather than looking for an alternative.
Catnaps. I just had a baby, and I've found that taking 10-15 minutes here or there (I set a phone alarm to make sure I wake up) helps me stay alert even though I'm somewhat sleep deprived. Sometimes I don't even fall asleep, but just shutting my eyes and allowing myself to ignore my surroundings does the trick and leaves me feeling refreshed.
Co-q-10 in the morning, about 12 to 14 hours before your usual/planned bedtime. It helps regulate the brain's chemical waking/sleeping cycle. It is the co-enzyme for melatonin and will increase production of melatonin roughly 12 hours after it is taken (for me, it seemed to work best if I took it about 14 hours before bedtime). In contrast, taking melatonin by itself will <i>not</i> increase production of co-q-10. However, if you have any trouble sleeping in the evening, you can also take a low dose of melatonin about 30 minutes before bedtime.<p>Co-q-10 is something we tend to be deficient in. If you are deficient, you won't sleep as well. This can help genuinely improve quality of sleep and help your baseline brain chemistry with regards to the waking/sleeping cycle. I have used co-q-10 in the morning combined with melatonin at night to quickly alter my sleep cycle by several hours starting three days or so before the needed change (when I had to get up unusually early or something). It works effectively without the issues caused by caffeine.<p>Best of luck.