The present invention regards an in-ear stimulation device (10, 10 10a) comprising at least one in-ear sensor (12, 14), a control unit (16), and an output transducer (18). The in-ear stimulation device (10, 10 10a) is configured to be worn at or on an ear (26) of a human user (28) and at least partly in an ear canal (30) of the user (28). The at least one in-ear sensor (12, 14) is arranged and configured to measure electrical activity signals (32) indicative of an electroencephalogram (EEG) of a user (28) wearing the device (10, 10 10a). The control unit (16) is configured to receive the electrical activity signals (32) and to detect (33) a presence of a slow oscillation (34), which is in a frequency range between 0.5 Hz and 1 Hz, in the electrical activity signals (32). The control unit (16) is further configured to trigger, in response to detecting (33) the presence of a slow oscillation (34), a generation of at least one stimulus impulse (36). The output transducer (18) is configured to receive the at least one stimulus impulse (36) and to generate a corresponding output sound (38).
