What a hard decision.

If you think asking for a massive flex time will help you do the job, then I would make sure they give it to you.

What a wonderful opportunity to help people though.

How long would you be helping in this position? A month? Two? If that's the case, then I would do it. But with your sleep problems, I wouldn't do it any longer. Maybe that should be a stipulation as well. Only for "X" amount of time.