A few specific ideas, that I realize will only fill a certain number of hours, but still very good use of your time:
1) bring a notebook and do planning for your startup - it will get you away from keyboard & screen all day, there's no such thing as a project that too well-planned out, and you don't need to worry about doing it on company time if everyone else is sleeping.
2) as someone mentioned, automate everything you do at work, so you can use those techniques/what you learned doing it to shorten your 5-6h at night to something reasonable.
3) no doubt people at work spend a lot of time on social media - so you do it to, only with a specific goal of networking. That could lead to anything.
Last thing - are you really building an MVP? I'll assume it's a web app of some type; I have no idea what it is, of course, but 5-6hrs a day is 3/4 full-time. You should only take a month or so to build and MVP at that rate, so maybe you haven't pared it down enough?