Sleeping through the night or through the nights?

There are substantial inter-individual differences in infants' longest consecutive sleep duration. However, intra-individual differences are rarely considered. The present study aimed to describe night-to-night variability in achieving 6 or 8 h of consolidated sleep over a 13-night period in 6-month-old infants.

Forty-four typically developing infants were part of the study (22 girls). When infants were 6 months old, mothers were asked to complete an infant sleep diary over 13 nights to measure the longest period of uninterrupted sleep each night. Two criteria were used to determine if infants were sleeping through the night: 6 and 8 h of uninterrupted sleep.

On average, mothers reported that their infant slept 6 h consecutively for about 5 nights out of 13. Nine infants (20.5%) never slept 6 h consecutively, three (6.8%) met the criterion every night, but most infants (n = 32; 72.7%) showed high variability between the nights. Mothers reported that their infant slept 8 h consecutively for about 3 nights out of 13. Half of the infants (50.0%) never slept 8 h consecutively, one infant (2.3%) slept 8 h consecutively every night, and twenty-one infants (47.7%) showed high variability.

These findings expand current knowledge by showing that there is not only high inter-individual variability, but also high intra-individual variability in infant sleep consolidation. Parents and clinicians should be aware that occasional sleeping through the night does not necessarily indicate a consolidation of this behavior.