Decreased slow-wave and paradoxical sleep in a rat chronic pain model

Diurnal sleep-wake patterns in the normal and the adjuvant arthritic rat were measured during the first 3 h of both light and dark periods. During the hours of maximal sleep in the normal rat, arthritic rats showed a significant increase in wakefulness (Wake), a shift to non-rapid-eye-movement (NREM) stages with lower amplitudes (LS and HS1), and a large reduction of NREM sleep with the highest-amplitude (HS2) and paradoxical sleep. Arthritic rats also showed marked sleep fragmentation manifested by more episodes of Wake, LS, and HS1 and shorter episodes of HS2 during both the light and the dark periods. Thus, arthritic rats cannot sustain long periods of sleep. In contrast to control rats, arthritic rats lacked a diurnal variation in Wake, total sleep, and electroencephalographic (EEG) delta activity. They also showed a decrease in overall EEG amplitude. In addition, there was a positive correlation between the severity of arthritis and the percentages of NREM sleep with low (LS) and moderate (HS1) amplitude. Thus, the decline in EEG amplitude could indicate a deficit of EEG generating mechanisms or some aspect of disease severity, such as pain.