Homeostatic and circadian processes are basic mechanisms of human sleep which challenge the common knowledge of large individual variations in sleep need or differences in circadian types. However, since sleep research has mostly focused on group measures, an approach which emphasizes the similarities between subjects, the biological foundations of the individual differences in normal sleep are still poorly understood. In the present work, we assessed individual differences in a range of EEG frequencies including sigma activity during non-REM sleep (8.0-15.5 Hz range) in a group of 10 subjects who had participated in a slow-wave sleep (SWS) deprivation study. We showed that, like a "fingerprint", a particular topographic distribution of the electroencephalogram (EEG) power along the antero-posterior cortical axis distinguishes each individual during non-REM sleep. This individual EEG-trait is substantially invariant across six consecutive nights characterized by large experimentally induced changes of sleep architecture. One possible hypothesis is that these EEG invariances can be related to individual differences in genetically determined functional brain anatomy, rather than to sleep-dependent mechanisms.