极夜，又称永夜，是在地球的两极地区，一日之内，太阳都在地平线以下的现象，即夜长为一个太阳日。

如果太阳直射点在哪个半球，另个一个半球的极地附近就会出现极夜现象。

极夜的范围与太阳直射点纬度有关，其边界与极点的纬度差就是太阳直射点的纬度。

所以： 春分过后，南极附近就会出现极夜，此后极夜范围越来越大；至夏至日达到最大，边界到达南极圈；夏至日过后，南极附近极夜范围逐渐缩小，至秋分日缩至0；秋分过后，北极附近出现极夜，此后北极附近的极夜范围越来越大；至冬至日达到最大，边界到达北极圈；冬至日过后，北极附近极夜范围逐渐缩小，至春分日缩至0。但是由于太阳折射原因，冬天极夜的范围不会完全到达极圈。

The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the Sun stays above the horizon for more than 24 hours. "Night" is understood as the center of the Sun being below a free horizon. Since the atmosphere bends the rays of the Sun, "light wins over darkness" by a few degrees. Hence the area that is affected by polar night is somewhat smaller than the area of midnight sun. The polar circle is located at a latitude between these two areas, at the latitude of approx. 66.5 degrees. For instance, in the most northern city of Sweden, Kiruna (located at the northern latitude of 67°51'), the polar night lasts for around 28 twenty-four-hour periods, while the midnight sun lasts around 50 twenty-four-hour periods.