Chhath puja has a special significance during the Sunrise and Sunset periods. Sunrise and sunset are the most important periods of the day during which a human body can safely get solar energy without any harm.
That is the reason why on third and fourth, devotees make offering when the sun is just setting and just rising, respectively.
During this period the solar energy has a low level of ultraviolet radiations so it is safe for the human body.
The ritual of Chhath puja provides mental calmness (by detoxifying the body and mind), enhances the energy level and immunity reduces the frequency of anger, jealous as well as a lot of negative emotions.
It is also believed that following the Chhath processes helps in slowing down the aging process.