[Cruncharoo]

they also found that going to sleep later correlated with higher morning alertness

I always found this to be true for myself--if I got 6 hours of sleep I'd be much more alert in the morning than if I got 8.

[sedeki]

Same here. But I always associated it with stress.

[xeromal]

I feel the same way but after wearing a heart rate monitor, it definitely detects a higher daily average heart rate so it can't be healthy.

Normal days: 50-55bpm

Low sleep days: 63-68bpm

[NathanielK]

I've noticed similar. I've woken up early, taken my pulse and notice it was a bit high and used that to decide to go back to sleep.

[_JoRo]

I've always feel like I get my best quality sleep when I go to bed extremely tired.

[sidpatil]

This could have to do with whether you're waking up in between versus in the middle of a sleep cycle. From what I understand, the former is preferable to the latter.