Hello.
The same way you'd tell her anything. If she wanted to eat pizza every night, and you wanted some variety, what would you say? I realize sex is more difficult to talk about than pizza, but the same principal applies.
You want to approach this in a positive way. In a loving and casual tone, you might say: I am SOOO loving our sex, but I'm noticing I'm falling asleep at work (or school). Just give me a little time to recharge my batteries, and I'll be jumping on you, you sexy thing! Dr. J
GET HER A VIBRATOR...IT WORKED