(A) Episodic word-pair task. Participants learned 120 word-nonsense word pairs. Nonsense words were 6–14 letters in length, derived from groups of common phonemes. During encoding trials (top left), word pairs were presented for 5 s. Participants completed the criterion training (top right) directly after encoding and received feedback after every trial. Recognition trials (bottom) were performed after a short delay (10 min, 45 trials) and again after a full night of sleep (10 hr, 135 trials).

(B) EEG power spectra during NREM sleep at electrode Cz for older (blue) and young (red) adults (mean ± SEM). Gray shaded areas indicate significant differences in low and sleep spindle frequency ranges. Insets depict topographical distribution of SO (<1.5 Hz; upper topographies) and sleep spindle (12–16 Hz; topographies on the right) power. Note that older subjects exhibited significantly reduced oscillatory power across the whole head.

(C) Top left: hypnogram (MT, movement time) from one exemplary of older subject and full-night multi-taper spectrogram at Pz (bottom left) with superimposed number of detected SO and sleep spindle events (white solid lines; 5 min averages). Top right: normalized circular histogram of detected spindle events relative to the SO phase. Note the peak in the right lower quadrant. Bottom right: peak-locked sleep spindle average across all detected events in NREM sleep (black). Low-pass filtered events (red) highlight that the sleep spindles preferentially peaked prior to the SO “up-state.” See also Figure S1 A.

(D) Exemplary young subject. Same conventions as in (C). Note that the sleep spindle amplitude is maximal after the SO peak.