How bad is it to only eat one meal a day and get about 5 hours of sleep each night?

In long-run? Unsusta. You r still young & so it may seem that this is normal to u. However, I suspect u will find that this pattern is unsustainable in the long-term. At some point you will build up a "sleep debt" and a caloric deficit. As u age u may find urself sleeping more, & eating more (even in that 1 meal). Those who eat 3x/day usually eat less overall than those who eat less frequently (but eat more per meal).
Bad. I doubt you are taking in enough nutrition to in that one meal to meet all your needs. Most people need more than 5 hours of sleep. Good sleep is needed to rest and refresh and people need sleep to be mentally sharp. You won't be able to keep it up forever. You are putting your physical and mental health at risk.