The annoyance-reaction is one of the central variables in noise research. After an introduction to different concepts and definitions of noise annoyance different scales of how noise annoyance can be measured are shown. The question is discussed whether disturbance effects of noise at different times of day are given. To clarify this problem, the results of a series of actual German noise studies are reported. In these studies differences between day- and night-time annoyance are found depending on the sound sources. For the case of road traffic noise no differences between day and night-time annoyance were found. In contrast, annoyance reactions are related to the time of day for railway and air traffic noise. Especially for aircraft noise, above a Leq of 50 dB(A) night-time annoyance rises faster than day-time annoyance. The effects are discussed in the frame of a cognitive model of noise annoyance. It is argued that annoyance judgements are based on an internal representation of the noise situation. Part of this representation are the event characteristics of the sound sources and their estimated impacts for disturbances at different times of day.