Phasic activation is part of the physiological sleep process. It occurs spontaneously in any sleep stage at any age and in any type of sleeping environment. Phasic activation can lead to sleep fragmentation and, therefore, have significant influences on sleep continuity. The sleeping physical environment can be quite variable depending on sleeper preferences or location of sleeping area. Thus, ambient temperature depends on both sleeper preferences and climatic conditions, while ambient noise depends on existing noise sources nearby. These external influences, and particularly loud noises, although often ignored, should not be underestimated as they can interact and profoundly disturb the sleep course, especially in older individuals. However, most of the studies done with usual noise exposure have clearly shown that the total amount of activation phases occuring during sleep is quite stable within an individual, and that almost no difference can be seen between disturbed and non-disturbed nights. Similarly, no difference can be seen in the total count of activation phases within an individual sleeping in different usual climatic conditions. Thus, it is necessary to reach very high noise exposure or be exposed to extreme climats to exhibit significant changes in the phasic activation pattern. Therefore, activation phase should be regarded as a very stable physiological sleep event, with strong individual characteristics, and quite resistant to the influence of ambient factors.