I think that the real answer is that we do not know the answer yet. It is likely that both aspects are correct to some degree, I'll explain my take as of now:

There is a real cost to sleeping less or sleeping too much and that some will feel the impact more than others also depending on the environment they are in (e.g increased stress, etc).

Depending on the combined variables in conjunction to lack of sleep, some may see positive gains by 'catching-up' on the weekend (think of parents for example), although it is likely that a marginal amount of lack of sleep will continue to accumulate over longer periods of time. This in turn can potentially lead to increased risk of heart attack and other issues.

The lack of sleep scenario can be seeing a little with the same lens as smoking, results can vary from person to person, but on average, it has a real negative impact when done often or under unfavorable circumstances.