Basalt temperature measurable even after sleeping without a duvet?

I'm currently looking into NFP. When measuring your temperature, you're supposed to measure your waking temperature, meaning you're still in bed after at least 4 hours of sleep.

Now, in this heat, I'm wondering: if I sleep without a blanket at night, will that affect the measurement? What would be the difference between getting up and measuring two minutes after getting up, and measuring in bed when you haven't used the blanket? Does that affect the measurement, or does it not matter? Can anyone help me?