Falling asleep in a car is a common phenomenon that is mainly due to the combination of both physical and psychological factors. One of the main reasons you may fall asleep in a car is your body's natural response to the rhythmic motion of the vehicle. As the car moves, your body is constantly adjusting to the changes in velocity and movement, and this can cause a feeling of sleepiness.
Another reason is that the low levels of oxygen within the car cabin, especially if the windows are closed or if the air conditioning is on, can also contribute to drowsiness. Furthermore, if you have been driving for long periods, or if you're on a long journey, mental fatigue, and boredom can also make you sleepier.
Additionally, if you're sleep-deprived or haven't had enough quality sleep in the days preceding the car journey, then it's more likely that you'll feel drowsy or sleepy while traveling. It's essential to take regular breaks, avoid driving when you feel excessively tired, and ensure that the air inside the car is well-circulated to prevent drowsiness.