Night Time is the right time for sleep? Are you kidding??

[Volcano Man]

Dirk Gross brings up a good point. I think that in these shorter scenarios, the main reason why you don't rest your units is because most of these scenarios begin with all units at 0 fatigue. This is not true with all scenarios since some do begin with varying levels of fatigue applied, but I think that most scenarios are designed (even if they are D+X into the campaign!) with full strength and full fatigue, simply because the designer may not have the time or want to expend the effort to go through and apply losses and fatigue to all units.