A Huffington Post report discusses sleep inertia, the transitional state of grogginess that typically follows waking up.
The brainstem arousal system is the part of the brain responsible for basic physical functioning. Though it’s activated immediately upon waking, our prefrontal cortex (PFC), which oversees decision-making and self control, takes awhile to get going. It can be up to 30 minutes for our PFC to catch up with the rest of our body.
During that time, our memory, reaction time, alertness and ability to perform various analytical tasks are not in full working order.