Ah the planes, gotta hate them
My standard operating procedure after 1941 and in the Med is to run surfaced by day and submerged at night until i get to the shipping lanes, when I only surface when I've run out of air or battery juice, and only long enough to replenish.
The idea is that before radar warning is available the planes will get you easier at night than by day, when you can at least see them coming and dive in time (hopefully)