Table 1.

Zero-order correlations, descriptive statistics and twin intra-class correlations

	1	2	3	4	5	6	7	8	9	10	11	12	MZ	DZss	DZos
1. Objective nighttime	–												0.84	0.47	0.43
Sleep duration (h)
2. Objective sleep	0.68***	–											0.84	0.50	0.43
Efficiency (%)
3. Objective sleep	−0.23***	−0.15***	-										0.76	0.68	0.57
Latency (min)‡
4. Objective sleep	−0.17**	−0.05	0.12**	–									0.95	0.97	0.86
Midpoint time§
5. Objective sleep	−0.20***	−0.01	0.08	0.40***	–								0.83	0.83	0.69
Midpoint time
Variability
6. Parent-reported sleep	0.36***	−0.02	−0.01	−0.22***	−0.24***	–							0.87	0.81	0.78
Duration (h)
7. Parent-reported	−0.05	0.05	0.09†	0.17***	0.09†	−0.16***	–						0.93	0.88	0.62
Daytime sleepiness||
8. Sex	0.15**	0.12**	0.01	0.03	0.02	0.04	0.09*	–
9. Age (years)	−0.22***	−0.02	−0.04	0.19***	0.25***	−0.26***	−0.20***	0.04	–
10. SES	0.18***	0.15***	0.06	−0.08	−0.08	0.09*	0.16***	0.07	−0.20***	–
11. Ethnicity	0.11*	−0.03	0.04	−0.15***	−0.12**	0.29***	−0.11*	−0.05	−0.10*	0.08*	–
12. Vacation	−0.05	−0.06	0.05	0.33***	0.13**	0.02	−0.04	−0.02	−0.08	0.08	−0.01	–
Mean¶	8.08	89.89	21.10	2:17	0.58	9.65	2.50	49.2	8.52	0.00	56.6	25.7
Standard deviation	0.74	5.91	15.93	46	0.30	0.86	1.72	–	0.63	0.66	–	–
Minimum	4.46	55.90	2.17	12:21	0.08	6.33	1.00	–	6.97	−1.20	–	–
Maximum	10.26	99.45	79.01	4:46	1.91	13.00	10.93	–	9.97	3.08	–	–
Skewness	−0.72	−1.37	1.82	0.64	1.24	−0.24	3.99	–	−0.21	1.12	–	–
Kurtosis	1.83	3.75	3.59	0.47	2.48	0.61	17.13	–	−0.09	2.01	–	–

N = 608. MZ, Monozygotic twin pairs; DZ_ss, same-sex dizygotic twin pairs; DZ_os, opposite-sex dizygotic twin pairs. Sex (1 = female), Race/ethnicity (1 = non-Hispanic European American), and Vacation (whether study was completed during a school vacation or during summer or not; 1 = study completed during a vacation) are reported to demonstrate sample demographics. Objective nighttime sleep duration, sleep efficiency, sleep latency, sleep midpoint time, and sleep midpoint variability were collected from each twin using wrist-based accelerometers during a week-long study protocol. Parent-reported nighttime sleep duration and daytime sleepiness were assessed using the CSHQ [26]. Objective sleep latency and parent-reported daytime sleepiness were windorized and log transformed for zero-order correlations and analyses given significant skew and kurtosis. SES was a standardized (z-scored) composite of primary caregiver highest education level, secondary caregiver highest education level, and family income to needs ratio at the eight-year assessment.

^‡Log transformed values used for zero-order correlations and raw windorized values used for descriptive statistics.

^§Sleep midpoint time presented on a 24-h time scale in descriptive statistics, with the mean, minimum, and maximum time representing AM, the standard deviation shown in minutes.

^||Log transformed values used for zero-order correlations and raw windorized values used for descriptive statistics.

^¶Mean for continuous variables and percentages of sample (for variables coded 1) for dichotomous variables.

†p < 0.10.

*p < 0.05.

**p < 0.01.

***p < 0.001.