Table 5. Associations between sleep problems and the occurrence of gastroesophageal reflux disease (GERD) among 210 monozygotic (MZ) twin pairs discordant for GERD.

		Co-twin within-pair analysis MZ twins
		Crude		Adjusteda
Exposure		OR	95% CI	OR	95% CI
Insomnia index b	Seldom	1.0	(reference)	1.0	(reference)
	Sometimes	1.4	0.9–2.2	1.3	0.8–2.2
	Often	1.6	0.9–2.8	1.5	0.9–2.7
Not rested when waking up	Seldom	1.0	(reference)	1.0	(reference)
	Sometimes	1.6	0.9–2.7	1.6	0.9–2.8
	Often	1.9	1.0–3.8	1.9	0.9–3.9
Disturbed sleep	Seldom	1.0	(reference)	1.0	(reference)
	Sometimes	1.8	1.0–3.4	1.9	1.0–3.6
	Often	1.5	0.7–3.2	1.4	0.7–3.1
Waking up too early	Seldom	1.0	(reference)	1.0	(reference)
	Sometimes	1.5	0.9–2.3	1.5	0.9–2.4
	Often	1.5	0.8–2.8	1.5	0.8–2.9

OR, odds ratio; CI, confidence intervals.

^aORs adjusted for educational level, body mass index, and tobacco smoking. Genetic and early environmental factors, sex and age are adjusted for by the within pair structure.

^bThe insomnia index was constructed by combining the three different sleep problem questions and 0 points were given for “seldom”, 1 point for “sometimes” and 2 points for “often”. The highest scores, 4–6 points or if the participant had answered “often”, were then classified as “often having sleep problems”, 1–3 points as “sometimes having sleep problems” and 0 as ”seldom having sleep problems”.