Yes, your line will naturally fluctuate throughout the night.
During sleep, your body still needs energy to carry out all its restorative and unconscious processes like breathing, muscle repair and brain activity.
When there is no glucose coming from external sources, your body gets this energy by switching to releasing energy from its stores; the liver and then fat cells. As it does during the day, your body works to keep your blood glucose in balance and allow it to provide the energy and resources your cells need to function, so there will be natural fluctuations throughout the night.