Sleep? WTF is that? I voted unpredictable.
I have never been a big sleeper as long as I can remember. My pattern hasn't changed much over the years. I'm 50 and I never get more than 4 or 5 hours of sleep at a time, without drugs, that is. I generally wake up with the sunrise, but rarely go to bed before 2AM or 3AM. Four to five hours is normal for me and my body violently rejected medical attempts to make me sleep more when I was in my 30s. I had succumbed to pressure from a live-in lover to try to "Get Help" for my sleeplessness. A couple of different drugs and some relaxation treatments screwed up my head for a while and I never really increased the amount of sleep I got, it just mad me feel bad all the time.
I believe that I just don't NEED as much sleep as some people do and even at my age, I'm still going strong. I often go around the clock without winding down enough for sleep. In fact about once a week I'm up all night listening to music or reading (or flying over the net) and I feel a little drowsy in the afternoon when I do that (now days), but forget trying to sleep if the sun's up. Of course, that's what drugs are for, right? To make the sun shut up.
I used to drink alcohol to wind down and if I was drunk enough I could get about 7 or 8 hours, but at a high price. I have learned that my favorite drug to help me get to sleep quicker is a single cup of coffee to take to bed. ONE CUP (1!) will soothe and warm my insides to an even glow and relax me enough to fall asleep much quicker. I avoid the second or third cup, though, because after 3 cups, for instance, I just get the shakes and sleep goes even farther away for several more hours.
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It's just music for me. The other stims don't work.