Individual stability of sleep spindle characteristics in healthy young males.

Objective: The purpose of the present analysis was to quantify the magnitude of intersubject and intrasubject variation of sleep spindle characteristics in a sample of healthy young males. Materials and methods: A total of 32 volunteers (age range 20-30 years) participated in a crossover study aimed at investigating possible effects of Terrestrial Trunked Radio (TETRA) signals on cognitive performance and human brain activity during sleep and waking. Sleep was polysomnographically assessed on nine identically organized study nights per individual. The inter- and intraindividual variability of automatically detected stage 2 non-rapid eye movement (NREM) sleep spindle (11-16 Hz) characteristics (number, density, duration, frequency, amplitude) derived from C3-A2 was determined. Stability of individual differences and the underlying variations were quantified using intraclass correlation (ICC) and coefficients of variation (CV), respectively. Note that there was no significant exposure effect on the considered sleep spindle characteristics. Results: All sleep spindle variables showed an ICC coefficient higher than 0.8, indicating almost perfect stability of interindividual differences. Interindividual variability differed strongly between the considered spindle parameters (CV range: 2-53 %). Intraindividual variability was for all parameters less pronounced (mean CV range: 1-24 %), but their ratios underline the high degree of individuality of each of the investigated parameters. Conclusion: The present analysis confirms the strong individual night-to-night stability of sleep spindle characteristics reported by previous studies. The results provide further evidence for treating sleep spindles as a possible trait, an issue which should be considered in studies relying on a parallel-group design. These data can also be used as reference values. [ABSTRACT FROM AUTHOR]