Is thoraco-abdominal phase relationship an indicator of sleep state?

The phase relationship between ribcage and abdominal movement in 1440 breaths from 12 infants (mean age 3.2 days) showed statistically significant differences between sleep states as judged by physiological criteria. The mean phase difference (+/- 1 SD) was 20 (+/- 16)% in active sleep, 9 (+/- 17)% in indeterminate sleep and 3 (+/- 5)% in quiet sleep (P less than 0.0005). However, the wide scatter within sleep states meant that even the mean value from 50 breaths was not specific enough to delineate sleep state.