  Objective Brain Age Index (BAI), calculated from sleep electroencephalography
(EEG), has been proposed as a biomarker of brain health. This study quantifies
night-to-night variability of BAI and establishes probability thresholds for
inferring underlying brain pathology based on a patient's BAI.
  Methods 86 patients with multiple nights of consecutive EEG recordings were
selected from Epilepsy Monitoring Unit patients whose EEGs reported as being
within normal limits. BAI was calculated for each 12-hour segment of patient
data using a previously described algorithm, and night-to-night variability in
BAI was measured.
  Results The within-patient night-to-night standard deviation in BAI was 7.5
years. Estimates of BAI derived by averaging over 2, 3, and 4 nights had
standard deviations of 4.7, 3.7, and 3.0 years, respectively.
  Conclusions Averaging BAI over n nights reduces night-to-night variability of
BAI by a factor of the square root of n, rendering BAI more suitable as a
biomarker of brain health at the individual level.
  Significance With increasing ease of EEG acquisition including wearable
technology, BAI has the potential to track brain health and detect deviations
from normal physiologic function. In a clinical setting, BAI could be used to
identify patients who should undergo further investigation or monitoring.
