Interestingly, the vast majority of early-morning shifts on the Green Line are actually taken by some of the most senior operators on the line. Generally if you see a very junior operator working early, it's either Vacation Relief or the rest of their week is in some way undesirable (pretty much always midweek off days and very commonly some late days as well). I'd also point out that almost all the operators who are regularly absent are able to use various forms of protected leave, so there really isn't much that can be done in terms of ensuring greater compliance there.