Here is an interesting study about the effect of mild sleep restriction and make up sleep. In this study, researchers took young, healthy volunteers and had them restrict their sleep from 8 hours down to 6 hours for 6 nights in a row. Then they let them sleep upto 10 hrs per night for 3 nights. During this study, they measured blood markers of inflammation, sleepiness, and performance with a computerized test.
The results showed that cortisol levels did not change with sleep restriction, but did fall below baseline after the recovery sleep. This is interesting to me on two fronts. One is that sleep deprivation did not raise cortisol levels. More interesting was the effect of recovery sleep on lowering cortisol from the baseline. I assume it's the relatively increased amount of deep sleep obtained during recovery that played a role in lowering cortisol levels.
Another marker for inflammation, interleukin-6, did increase during sleep restriction and fell back to baseline with recovery sleep. And subjective and objective sleepiness increased after restriction but returned to baseline after recovery. However, performance deteriorated significantly after restriction and did not improve completely after recovery. And this was after just one week of restriction. It will be interesting to see long term studies that would replicate this week after week, like what many working people do. Would performance continue to decline and not fully recover on weekends? Or would acclimation happen, such that performance level decrements stopped or where recovery sleep became better at improving performance?