Altered states of consciousness (ASC) are marked, temporary, reversible deviations in subjective experience or psychological functioning from usual waking consciousness, distinct from psychiatric disorders. Breathing is not only essential physiologically but also modulates brain oscillatory activity. Studies show that reducing breathing to ~6 breaths/min increases delta (1-4 Hz) and theta (4-8 Hz) power, especially in prefrontal and limbic regions, and enhances default mode network (DMN) connectivity. These changes are linked to introspection and ASC, suggesting slow breathing as a non-invasive way to modulate consciousness. This study investigates how slow breathing modulates consciousness through EEG analyses: power spectral density (PSD), complexity metrics, and functional connectivity. A within-subject design will be used with 40 healthy adults (18-50 years). Each will undergo two conditions in randomized order, two weeks apart: (1) experimental, 5 min baseline then 15 min guided slow nasal breathing at 4 breaths/min; (2) control, 5 min baseline then 15 min spontaneous breathing. EEG (32 channels, Brain Vision) and respiratory belts will monitor neural and breathing activity. Subjective ASC will be assessed with the TAS, PCI, and 11D-ASC. This study will provide evidence on slow breathing as a neuromodulation tool to modulate brain activity and induce ASC.