Sleep intensity in fibromyalgia: focus on the microstructure of the sleep process

Alpha electroencephalography (EEG) predominance has been described during sleep in patients suffering from the fibromyalgia syndrome (FMS). However, EEG power density in the lower frequency bands probably better reflects the restorative functions of sleep. This study was conducted to describe the energy in all frequency bands in the sleep EEG. Ambulatory sleep recordings were performed on 12 women with FMS and 14 control women. Epochs were classified according to standard criteria. Moreover, all 2-s segments (n = 287,355) of the EEG in non-rapid-eye-movement (NREM) 2-4 sleep were subjected to frequency analysis using autoregressive modelling. Frequency bands were: delta (0.5-3.5 Hz), theta (3.5-8 Hz), alpha (8-12 Hz), sigma (12-14.5 Hz) and beta (14.5-25 Hz). In patients with FMS, there was a predominance of EEG power in the higher frequency bands [two-way analysis of variance (ANOVA), alpha: P = 0.043; sigma: P = 0.004] at the expense of the lower frequencies (ANOVA, delta: P = 0.005; theta: P = 0.008). The same trends were obtained for the individual sleep cycles. The calculations of total delta power in the time domain showed an exponentially declining curve in healthy subjects, but a flatter decline in FMS. The decreased power in the low-frequency range might reflect a disorder in homoeostatic and circadian mechanisms during sleep and may contribute to daytime symptoms in patients with fibromyalgia.