Table 1. Characteristics of cross-sectional and cohort studies included in the meta-analysis.

Author study name	Country observation period	No. of participants	No. of cases	Age range mean (s.d.)	Gender population	Metabolic syndrome assessment	Sleeping assessment	Sleep (h)	Odds ratio (95% CI)	Adjusted covariates
Cross-sectional studies
Santos et al.23	Portugal 1999–2003	832	96	18–92	Men	NCEP ATP-III	Questionnaire	⩽6	1.40 (0.76–2.60)	1, 3, 4, 5
			216					7	1.00 (Reference)
			214					8	1.10 (0.73–1.70)
			227					⩾9	1.50 (0.90–2.60)
		1332	153		Women			⩽6	0.92 (0.55–1.50)
			270					7	1.00 (Reference)
			401					8	1.20 (0.80–1.70)
			388					⩾9	2.00 (1.30–3.00)
Choi et al.33 KNHNS, 2001	Korea 2001	633	217	44.1 (0.4)	Both	Modified NCEP ATP-III	Questionnaire	⩽5	1.20 (0.87–1.60)	1, 2, 3, 4, 5, 6, 12
		1056	293					6	1.10 (0.85–1.30)
		1182	274					7	1.00 (Reference)
		1056	293					8	1.30 (1.00–1.70)
		296	94					⩾9	1.70 (1.20–2.50)
Hall et al.36 AHAB	USA	187	58	30–54	Both	AHA/NHLBI	Interview	<6	1.80 (1.20–2.80)	1, 2, 4, 8, 9, 10, 14
		402	101					6–6.99	1.50 (1.10–2.10)
		525	100					7–8	1.00 (Reference)
		100	25					>8	1.80 (1.00–3.10)
					Not taking antihypertensive medication			<6	1.80 (1.10–2.70)
								6–6.99	1.50 (1.00–2.10)
								7–8	1.00 (Reference)
								>8	1.60 (0.91–3.00)
Arora et al.22 GBCS	China	1142	329	50–96	Both	Modified NCEP ATP-III	Interview	<6	0.97 (0.88–1.10)	1, 2, 3, 4, 5, 8, 11
		2020	570					6–7	1.00 (0.93–1.10)
		2303	603					7–8	1.00 (Reference)
		1995	575					8–9	1.20 (1.10–1.30)
		762	226					⩾9	1.20 (1.10–1.30)
					Healthy			<6	0.93 (0.84–1.00)
								6–7	0.98 (0.90–1.10)
								7–8	1.00 (Reference)
								8–9	1.10 (1.10–1.20)
								⩾9	1.20 (1.10–1.30)
				51–61	Middle age			<6	1.10 (0.91–1.20)
								6–7	1.00 (0.92–1.10)
								7–8	1.00 (Reference)
								8–9	1.10 (1.00–1.30)
								⩾9	1.30 (1.20–1.50)
				>61	Old age			< 6	0.93 (0.83–1.10)
								6–7	0.98 (0.89–1.10)
								7–8	1.00 (Reference)
								8–9	1.20 (1.10–1.30)
								⩾9	1.10 (0.95–1.30)
Kobayashi et al.17	Japan 2008	7295	641	44.8 (12.8)	Both	JASSO	Questionnaire	<6	1.40 (1.20–1.60)	1, 2, 4, 6, 13
		11355	905					6–6.99	1.10 (0.95–1.30)
		6732	592					7–7.99	1.00 (Reference)
		2410	233					⩾8	0.98 (0.83–1.20)
Najafian et al.35 IHHP	Iran 1999–2006	1447	485	38.89 (14.93)	Both	NCEP ATP-III	Interview	⩽5	1.50 (1.30–1.70)	1, 2
		2336	575					6	1.20 (1.10–1.40)
		7622	1654					7–8	1.00 (Reference)
		1089	223					⩾9	0.79 (0.68–0.94)
		5976			Men			⩽5	1.30 (0.99–1.60)	1
								6	1.10 (0.89–1.30)
								7–8	1.00 (Reference)
								⩾9	0.77 (0.55–1.10)
		6320			Women			⩽5	1.70 (1.40–2.00)
								6	1.30 (1.10–1.50)
								7–8	1.00 (Reference)
								⩾9	0.81 (0.67–0.98)
		10699		<60	Both			⩽5	1.70 (1.50–2.00)	2
								6	1.20 (1.10–1.40)
								7–8	1.00 (Reference)
								⩾9	0.84 (0.70–1.00)
		1597		>60	Both			⩽5	1.10 (0.84–1.40)
								6	1.10 (0.79–1.40)
								7–8	1.00 (Reference)
								⩾9	0.61 (0.40–0.92)
Sabanayagam et al.19 NHANES	USA 2005–2008	935	377	44.63 (0.46)	Both	AHA-NHLBI	Questionnaire	⩽5	1.20 (0.98–1.60)	1, 2, 3, 4, 5, 8, 10
		1412	548					6	1.10 (0.93–1.30)
		1777	650					7	1.00 (Reference)
		1621	579					8	0.92 (0.76–1.10)
		377	135					⩾9	0.97 (0.69–1.40)
McCanlies et al.34	USA	28	7	39.61	Both	NCEP ATP-III	Questionnaire	< 6	2.30 (0.81–6.50)	1, 2, 3, 4
		70	7		Police			⩾6	1.00 (Reference)
Wu et al.20	Taiwan	954	198	47.1 (12.0)	Men	Modified NCEP ATP-III	Questionnaire	<6	1.30 (1.00–1.60)	1, 3, 4, 5, 7
	2006–2009	3202	535	44.8 (11.1)				6–8	1.00 (Reference)
		142	27	52.3 (12.2)				> 8	1.40 (0.82–2.50)
		794	94		Women			< 6	1.00 (0.72–1.50)
		1909	172					6–8	1.00 (Reference)
		99	9					> 8	0.90 (0.32–2.50)
Yoo et al.21	USA	32	10	22–60	Both	Modified AHA-NHLBI	Questionnaire	⩽6	2.30 (0.71–7.50)	1, 2, 4, 8, 17
		53	13		Police			>6–<8	1.00 (Reference)
		21	12					⩾8	1.90 (1.30–18.0)
Cohort studies
Choi et al.15 KGRC	Korea 2005–2009	27	5	40–70	Men	Modified NCEP ATP-III	Interview	< 6	0.62 (0.24–1.60)	1, 4, 5, 7, 8
		209	49					6–7.9	1.00 (Reference)
		132	23					8–8.9	0.74 (0.44–1.20)
		18	5					≥10	1.60 (0.61–4.00)
		68	20		Women			<6	1.80 (1.10–3.00)	1, 4, 5, 7, 8, 15
		403	61					6–7.9	1.00 (Reference)
		226	34					8–8.9	0.91 (0.59–1.40)
		24	7					⩾10	1.70 (0.71–3.90)
Otsuka et al.18	Japan 2005–2009	120	20	35–63	Men	The Japanese criteria	Questionnaire	⩽5	3.20 (1.50–6.60)	1, 4, 5, 6, 16
		559	46					5–6	1.80 (0.96–3.50)
		269	13					>6	1.00 (Reference)

Abbrevations: AHA-NHLBI, American Heart Association/National Heart Lung and Blood Institute; AHAB, Adult Health and Behavior; CES-D, Center for Epidemiological Studies Depression Scale; CI, confidence interval; GBCS, The Guangzhou Biobank Cohort Study; IHHP, Isfahan Healthy Heart Program; JASSO, Japan Society for the Study of Obesity; KGRC, The Korean Genomic Rural Cohort; KNHNS, Korean National Health and Nutrition Survey; NCEP ATP-III, National Cholesterol Education Adult Treatment Panel III; PSQI, Pittsburgh Sleep Quality Index; PSS, Perceived Stress Scale; 1, age; 2, sex; 3, education; 4, smoke; 5, alcohol; 6, exercise; 7, body mass index; 8, physical activity; 9, race; 10, depression; 11, diagnosed mental illness, insomnia, use of hypnotics, daytime sleepiness, snoring, mean systolic pressure, glucose, total cholesterol and triglycerides; 12, family history of hypertension or diabetes, residential area and monthly income; 13, myocardial infarction and cerebral infarction; 14, low-density lipoprotein cholesterol; 15, menopause; 16, frequency of vegetable intake, frequency of oily food intake and frequency of salty food intake; 17, burnout, Center for Epidemiological Studies Depression Scale and Perceived Stress Scale.