Just my belief here and it may not be accurate. But I think that once you focuse more on dream recall you're changing your entire sleeping patterns, You're now extending also Non-Rem periods this is the time when your body actually does rest, this being the reason why you are not as tired as before.

Now that your sleep patterns has pretty much become habitual you're able to differentiate the days when you can sleep in more. I usually sleep longer myself on weekends because I work M-F. This is because my Body realize the 5 days that I wake up at a certain time and the last 2 days I can sleep in, this is automatic. You pretty much set your sleep habits into a time format where your body automatically adjust to. Hope that helped Krippie.