Brahmi Muhurta - Scientific reason of sages getting up early in the morning, at a particular time :

The time 90 minutes prior to sunrise is called Braahmi Muhurtham. This time is good for academics, purohits and the time where in the later hours, in the night, the last 48 minutes. (2 ghadi) is called Braahmi Muhurtham. 'Braahmi' means Saraswathi, the Goddess of intellect. The above specified time is auspicious for gaining and sustaining intelligence and knowledge, owing to which it is called Braahmi Muhurtham.

It is during Braahmi muhurtham that Sun god spreads his rays just as a peacock spreading its feathers. He starts spreading his light and energy throughout the world. The light rays from the Galaxies influence the human brain. The nascent sun spreads thousand arms in the form of rays across the sky, which emit light-blue devine rays. These rays bring to life the cells and the brain. Lord Surya or the sun god is also the god of life. If the man can synchronise his senses with these rays during this hour he will be empowered with unchallengeable energy. This observation was endorsed by sages. This is the time when the life under the sun still remains in deep sleep supported by the tranquil and pleasant environment, the sages and munis spread the power of penance, which comes out in the form of high powered electrical and magnetic charge, for the upliftment of the living creatures on the earth. If one keep awake during these hours, it is possible to benefit from this charge.

There are other advantages of waking up at Braahmi Muhurtham. Throughout the night the wind takes unimaginable amounts of Amrutham (elixir) from the rays of the moon and the stars. This wind provides a healthy cover in the atmosphere at dawn. The air during Braahmi Muhurtam is called 'Veeravayu'. The Veeravayu helps the body maintain good health, increases the facial glow, inspires the mind and sharpens the intellect. At dawn, it is said, manes in the other world are at peace and gain strength. This strength spreads across the world.

James Allen is an American scientist. He opines that "each of you think scientifically rising early is good for health. It increases blood circulation. Body gains energy. This energy helps him spend the entire day enthusiasticallly. Waking at down blossoms the intellect. Solution can be found for any kind problems since one can examine the problem analytically during those hours. The brain remains calm and whatever is read is firmly understood and
remembered.

Early in the mornings the weather is tranquil. Oxygen is present in greater ratio. Lungs get healthy air. When the warm rays of the sun fall on the skin there will be no skin diseases. Sweat pores will be activated. Digestion will improve. Thus say physiologists.