The Sun's brightness slightly varies with its activity cycle, but only on the order of about 0.1% at most. This measurement is termed Total Solar Irradiance. The brightness is also cyclical at certain wavelengths to a higher degree, which is due to the presence or absence of sunspots/active regions. On billion year timescales, the Sun's brightness will increase quite a bit as it evolves along the main sequence track on the Hertzsprung-Russell diagram, but the Sun is in a very stable stage of its life and humans have not experienced significant inherent brightness increases (although axial changes of the Earth due to precession do affect the distribution of energy across the globe).