Getting through a 7.40 first period on one cup of chai a day

My doctor suggested cutting back, so I've gone from three cups to one. Fine in theory. In practice I'm standing in front of my DP class at 7.40 trying to explain something I understood perfectly well the evening before. Nobody at school knows yet, so I can't really explain the yawning. My 4 year old has started asking why I lie down on the sofa the minute I get home. What got you through first period on less caffeine? Other than an earlier bedtime, which with a 4 year old is optimistic.