A sidereal day is approximately one full day which is one full rotation but 4 minutes shorter than a solar day which lasts 24 hours. This means that the north and south poles receive only half the sunlight each, due to the way the earth tilts and the direction the sun's light is facing. This is why some countries are very hot because they are directly exposed to the sun and have longer days because the sun takes longer to leave. Due to the seasons, the length of day and night may vary by a few hours. When you see the sun shining in the sky it is day and when you don't see the sun shining in the sky it is night. This is why we see the sun move across the sky starting from the east in the morning and moving west in the evening. The sun also moves in the sky because the earth rotates around its axis. In this diagram it shows the earth's day and night, against the sunlight shining directly on one side of the earth