According to Huygens principle,light travels in the form of wave fronts.

So why do we have nights though the light from sun should brighten our dark side as it is larger in size.

Its wavefronts which act like a new source of light is providing us with intensiy.

When applied to the propagation of light waves, this principle states that: Every point on a wave-front may be considered a source of secondary spherical wavelets which spread out in the forward direction at the speed of light. The new wave-front is the tangential surface to all of these secondary wavelets.