1. A wrist-worn apparatus, comprising: at least one physical activity sensor to measure physical activity data related to a user of the wrist-worn apparatus; and at least one processing unit and at least one memory including a computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processing unit, to cause the wrist-worn apparatus to perform operations comprising: supporting a normal operation mode during which functionalities of the wrist-worn apparatus are available through an operating system; supporting a night operation mode during which the wrist-worn apparatus is configured to perform a sleep analysis based on executing a physical activity algorithm for the physical activity data, wherein the physical activity algorithm applies a direct low-level hardware access; and switching between the normal operation mode and the night operation mode, wherein the at least one processing unit comprises: a low power processing unit configured to run the sleep analysis and the physical activity algorithm during the night operation mode; and a high power processing unit configured to run the operating system during the normal operation mode, wherein the low power processing unit has a lower power consumption in performing a given task than the high power processing unit, and wherein a switch to the night operation mode comprises: activating the low power processing unit to run the sleep analysis and the physical activity algorithm; and inactivating the high power processing unit.