Table 1.

Twin studies of insomnia phenotypes

Authors	Sample	Phenotypes	Heritability
Webb & Campbell 1983	14 MZ, 14 DZ Young adults	Sleep latency	N/A
		Wake time
Partinen et al 1983	2238 MZ, 4545 DZ Adults	Sleep length	h2 = .44
		Sleep quality	h2 = .44
Heath et al 1990	1792 MZ, 2101 DZ Adults	Sleep quality	h2 = .32
		Initial insomnia	h2 = .32
		Sleep latency	h2 = .44 ♂, .32 ♀
		Anxious insomnia	h2 = .36
		Depressed insomnia	h2 = .33
Heath et al 1998	1792 MZ, 2101 DZ Adults	Composite score	12.1% of variance in ♀, 8.3% in ♂
McCarren et al 1994	1605 MZ, 1200 DZ Male veterans	Trouble falling asleep	h2 = .28
		Trouble staying asleep	h2 = .42
		Waking up several times	h2 = .26
		Waking up tired	h2 = .21
		Composite score	h2 = .28
De Castro 2002	86 MZ, 129 DZ Adult ‘good sleepers’	Sleep duration	h2 = .30
		Number of wakeups	h2 = .21
Watson et al 2006	1042 MZ, 828 DZ Young adults	Insomnia	h2 = .64
Boomsma et al 2008	548 twins, 265 siblings Adults	Insomnia factor	h2 = .20
Gregory et al 2004	2162 MZ, 4229 DZ Age 3–7 years	Sleep problems scale	h2 = .18 ♂, .20 ♀
Gregory et al 2006	100 MZ, 200 DZ Age 8	Sleep onset delay	h2 = .17 for child report, .79 for parental report
		Night wakings	h2 = .27 for child report, .32 for parental report
Gregory 2008	100 MZ, 200 DZ Age 8	Dyssomnia scale	h2 = .71
Gregory et al 2006	192 MZ, 384 DZ Age 8	Sleep problems score	h2 = .61