For years, I got by on 6 hours a night, thinking that was all I needed. In a moment of clarity, I realized that my creativity and problem-solving skills had declined. I took a 2-week vacation, determined that it would be a "do-nothing" one where I did whatever I wanted. It was not to be a scheduled, rushed time off, but one where I would not set the alarm. I also chose to use a set bedtime every night. My sleep hours slowly lengthened, and by the end of the two weeks, I was waking up at almost exactly 8 hours on the dot.
I have since planned my sleep accordingly: a set bedtime that allows 8 hours of sleep before I need to get up for my work day. I keep the same bedtime on weekends, but do not set an alarm, and I still wake up at almost 8 hours on the dot.
Much to my surprise, I am more productive, feel less stressed, and have been able to meet deadlines with ease. My supervisees have even noted and commented on my more relaxed behavior and appearance.
I don't advocate 8 hours for everyone, but I would recommend that you work on discovering your own optimum sleep schedule. It makes a world of difference in GTD.
Carolyn