A sleep EEG was recorded in 44 patients whose waking EEG showed epileptic activity. The time in seconds in which one epileptic discharge occurred was determined. Considering the waking and sleep EEGs together one epileptic discharge occurred every 7.8 seconds. Focal epileptic activity was recorded nearly twice as frequently (1 paroxysm every 4.6 sec.), as generalized activity (1 paroxysm every 9.8 sec.) or generalized activity with lateral emphasis (1 paroxysm every 8.8 sec.). Considering the sleep and waking EEGs separately it was found that on average more epileptic activity occurred in sleep than in the waking EEG (6.4 vs. 10.8 sec.). There was no difference between generalized (8.9 vs. 13.5 sec.), generalized activity with lateral emphasis (7.7 vs. 12.9 sec.) and focal activity (3.9 vs. 7.6 sec.). Comparing, however, the frequency of epileptic activity in each individual patient during wakefulness and sleep it was seen that during sleep more activity was found in only 56.8% of the patients. If a difference of 3 seconds is considered to be due to chance, then more epileptic activity was found in 34.1% of the patients during sleep and in 24.5% during wakefulness. Thus, the sleep EEG by no means always activates epileptic activity.
