That's supposedly one of the most important ways of motivating students -- letting them see how you love this thing they're studying (that they may come into class having not so much affection for). And I do love it, thankfully (Joyce and Yeats today, woohoo!), so I try to pass that along. But it's tiring. Teaching really is a lot like acting, so if you can imagine performing in a play for an hour and a half, then taking a break for lunch, then doing another hour and a half, then walking to another location across campus, and then performing for one more hour and a half -- well, that's what it's like. And doing the same material over and over again doesn't help.
But there are some mornings where I actually wake up on my own, having slept enough. Like today. And oh, the day is so much better. Still tiring by the time I get home, and Thursdays are particularly rough because Kevin teaches in the evening, so when I get home, I stay on duty with the kids from 4 - 8, and then at 8, I go straight to bed. Essentially, a 15 - 16 hour work day. But sleep makes it possible.
Which is all a roundabout way of saying, "Dearest Anandan, thank you for sleeping until 5:15 today. (Instead of your more typical 4:30, or 4:00, or [shudder] 3:30.) Mama loves you so much."
I absolutely agree with your description of what teaching is like. I REALLY wonder how high school teachers keep it up for so much longer than we do each week. I tend to think of the good ones as superhuman.