Table 2.

Baseline Characteristics (Frequencies) of US Women From the Nurses’ Health Study II Cohort, by Quartile of Urinary 6-Sulfatoxymelatonin^a:Creatinine Ratio, 1997–1999

Covariate	Total Population (n = 1,075)		Quartile of Urinary aMT6s:Cr Ratio								P Valueb
			1 (n = 268)		2 (n = 269)		3 (n = 269)		4 (n = 269)
	No.	%	No.	%	No.	%	No.	%	No.	%
Insulin resistancec	268	25	87	32	74	28	62	23	45	17	0.0002
No shift work in past 2 years	893	83	217	81	219	81	220	82	228	85	0.65
Family history of diabetes mellitus	166	15	37	14	41	15	51	19	38	14	0.32
Smoking status
Current smoker	54	5	15	6	17	6	10	4	12	4	0.52
Past smoker	246	23	62	23	62	23	59	22	63	23	0.98

Abbreviation: aMT6s:Cr, 6-sulfatoxymelatonin:creatinine.

^a 6-Sulfatoxymelatonin is the major metabolite of melatonin.

^b P values were calculated by means of the χ² test.

^c Insulin resistance was defined as the lowest 25% of insulin sensitivity index values, as measured by the McAuley formula (20).