This is taken from island in Päijänne, the second largest lake in Finland. Like most of the lakes in Finland, due to ice ages, its highly irregular in shape and coastline is very fractalic.
Unlike most lakes, Päijänne has a lot of rock formations on its coasts.
The image has been taken around 03:40, so the bluest and darkest time of the night has gone already, and it's already quite light again.