Lights are dimmed right down as per CAA recommendation (regulation?) for all night takeoffs and landings. This is because one's eyes are very sensitive to changes in light levels at night (it takes about 20mins to fully adjust to a change in white light level at night), and if one were to be involved in an accident/ evacuation in the dark, it would be preferable that your eyes are already accustomed to the lower levels of light typically experienced outside the aircraft. Hope that helps explain things