Rotating Night Shift Work and Mammographic Density

Beata Peplonska; Agnieszka Bukowska; Wojciech Sobala; Edyta Reszka; Jolanta Gromadzinska; Wojciech Wasowicz; Jenny Anne Lie; Helge Kjuus; Giske Ursin

doi:10.1158/1055-9965.EPI-12-0005

DOI: 10.1158/1055-9965.EPI-12-0005 Published July 2012

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Table 1.

Selected characteristics of the nurses and midwives with screening mammography in the cross-sectional study

	Total population (n = 640)		Rotating nights (n = 321)		Day shifts (n = 319)
Characteristics	n (%)	Mean (SD)	n (%)	Mean (SD)	n (%)	Mean (SD)
Age,^a y		49.0 (5.4)		48.1 (5.2)		50.0 (5.4)
Menopausal status
Pre	368 (57.5)		206 (64.0)		162 (51.0)
Post	272 (42.5)		115 (36.0)		157 (49.0)
Age at menopause (in postmenopausal),^b y		49.3 (3.9)		48.8 (3.9)		49.6 (4.1)
Age at menarche,^b y		13.2 (1.5)		13.3 (1.4)		13.0 (1.6)
Number of full-term births
0	76 (11.9)		39 (12.1)		37 (11.6)
1	215 (33.6)		101 (31.5)		114 (35.7)
2	313 (48.9)		156 (48.6)		157 (49.2)
3–4	36 (5.6)		25 (7.8)		11 (3.5)
Age at first full-term birth (≥37 wk), y
≤24	319 (56.6)		159 (56.4)		160 (56.7)
>24–<30	188 (33.3)		91 (32.3)		97 (34.4)
≥30	57 (10.1)		32 (11.3)		25 (8.9)
Family history of breast cancer^b
No	587 (91.7)		292 (91.0)		295 (92.5)
Yes	51 (8.0)		27 (8.4)		24 (7.5)
BMI, kg/m²		27.2 (4.7)		27.1 (4.7)		27.2 (4.7)
Smoking
Current	188 (29.4)		106 (33.0)		82 (25.7)
Past	173 (27.0)		77 (24.0)		96 (30.1)
Nonsmokers	279 (43.6)		138 (43.0)		141 (44.2)
Ever oral contraceptives or sex hormones use
Yes	153 (23.9)		74 (23.0)		79 (24.8)
No	487 (76.1)		247 (77.0)		240 (75.2)
Calendar season of mammography
Summer	135 (21.1)		63 (19.6)		72 (22.6)
Autumn	251 (39.2)		129 (40.2)		122 (38.2)
Winter	101 (15.8)		47 (14.6)		54 (16.9)
Spring	153 (23.9)		82 (25.6)		71 (22.3)
Total duration of the night shift work,^a y
≤5	77 (12.0)		3 (0.9)		74 (23.2)
>5–≤15	163 (25.5)		26 (8.1)		137 (43.0)
>15–≤25	234 (36.6)		145 (45.2)		89 (27.9)
>25	166 (25.9)		147 (45.8)		19 (5.9)
Arithmetic mean − percentage of mammographic density (SD)		23.2 (20.0)		24.5 (20.4)		22.0 (19.6)
Arithmetic mean − absolute area dense, cm² (SD)		28.1 (26.9)		30.0 (28.4)		26.2 (25.2)

↵^aP < 0.05 for the difference in means between the current rotating night shift nurses and non-shift nurses.
↵^bMissing information for age at menopause in one person with rotating night shifts and one person on day shift; age at menarche, one night shift and two day shifts; family history was missing on 2 nurses working rotating night shifts.

Table 2.

Percentage and absolute mammographic density by selected characteristics of the rotating night shift work

Night shift work characteristics	n	Percentage of mammographic density Adjusted^a mean (95%CI)	Absolute density, cm² Adjusted^a mean (95%CI)
Current rotating night shift
No	319	22.5 (20.8–24.3)	21.8 (19.4–24.3)
Yes	321	23.6 (21.9–25.4)	23.9 (21.4–26.4)
P		0.62	0.25
Total duration of night shift work, y
≤5	64	22.5 (18.8–26.1)	23.3 (17.8–28.8)
>5–≤15	165	24.5 (22.1–27.0)	23.8 (20.3–27.2)
>15–≤25	206	21.5 (19.4–23.7)	21.9 (18.8–24.9)
>25	205	23.7 (21.4–26.0)	23.1 (20.0–26.2)
P_trend		0.93	0.78
Total number of hours of night shift work by tertiles^b
I	214	24.0 (21.8–26.1)	23.8 (20.7–26.9)
II	214	22.3 (20.2–24.5)	22.5 (19.5–25.5)
III	212	23.0 (20.7–25.2)	22.3 (19.3–25.4)
P_trend		0.49	0.49
Cumulative number of night shifts
≤999	185	23.8 (21.5–26.1)	23.6 (20.3–26.9)
1,000–1,999	225	22.5 (20.5–24.6)	22.9 (19.9–25.8)
2,000+	230	23.1 (20.9–25.2)	22.3 (19.4–25.2)
P_trend		0.66	0.56
Average frequency (weighted by duration); no. of night shifts/mo
≤2	214	24.2 (22.0–26.3)	24.4 (21.3–27.5)
>2–≤3.9	213	22.3 (20.3–24.4)	22.8 (19.8–25.8)
>3.9	213	22.7 (20.6–24.8)	21.5 (18.6–24.4)
P_trend		0.33	0.18
Total duration of reported exposure to surgical lamp, y
Day shifts (reference)	550	23.3 (21.9–24.6)	23.0 (21.1–24.9)
>0–≤10	39	21.3 (16.8–25.9)	21.1 (14.3–27.8)
>10	51	22.7 (18.4–26.9)	23.2 (17.1–29.4)
P_trend		0.61	0.92

↵^aAdjustment for age, BMI, menopausal status, age at menopause, age at menarche, smoking, and season of the year.
↵^bI tertile, ≤13,056 hours; II tertile, >13,056 to ≤24,304 hours; and III tertile, >24,304 hours.

Table 3.

Percentage of mammographic density and absolute density by selected characteristics of the rotating night shift work

Current rotating night shift work characteristics	n	Percentage of density Adjusted^a mean (95%CI)	Absolute density, cm² Adjusted^a mean (95%CI)
Current night shift work frequency per month
Day shifts (reference)	319	22.5 (20.8–24.3)	21.8 (19.4–24.3)
2–7 nights	285	23.6 (21.8–25.5)	23.9 (21.2–26.5)
8+ nights	36	23.7 (18.4–29.0)	24.2 (16.7–31.7)
P		0.40	0.27
Night shifts working hours,^c h
Day shifts (reference)	319	22.5 (20.7–24.2)	21.8 (19.4–24.2)
<10	84	23.2 (19.9–26.5)	22.7 (18.0–27.4)
≥10–14	230	23.8 (21.7–25.9)	24.3 (21.3–27.3)
P		0.34	0.20
Light at work^c
Day shifts (reference)	319	22.5 (20.7–24.2)	21.8 (19.4–24.2)
Dim	139	23.0 (20.4–25.6)	23.0 (19.3–26.7)
Full light	140	24.4 (21.8–27.1)	25.1 (21.2–28.9)
Surgical lamp	35	22.8 (17.7–28.0)	22.8 (15.3–30.2)
P_trend		0.34	0.24
Activities at work during the night shift^c
Day shifts (reference)	319	22.5 (20.7–24.2)	21.8 (19.4–24.2)
1^b	23	26.8 (19.9–33.7)	25.4 (15.9–35.0)
1 and 2 or 2	221	23.1 (21.1–25.2)	23.3 (20.4–26.3)
Any combination with 3	70	24.1 (20.4–27.8)	25.1 (19.6–30.6)
P_trend		0.43	0.24

↵^aAdjustment for age, BMI, menopausal status, age at menopause, age at menarche, smoking, and season of the year.
↵^bCategories of the activities: 1, taking care of patients, injections, administering pills; 2, small interventions; and 3, assistance in operations (deliveries).
↵^cMissing information for 7 women.

Table 4.

Percentage and absolute mammographic density by morning MT6s levels

	Morning MT6s^b
	Q1 (n = 160)	Q2 (n = 160)	Q3 (n = 160)	Q4 (n = 160)	P
Percent mammographic density [adjusted^a mean (95%CI)]	22.6 (20.1–25.1)	23.1 (20.7–25.6)	23.2 (20.8–25.5)	23.4 (21.0–25.9)	0.65
Absolute density [adjusted^a mean (95%CI)]	21.8 (18.3–25.2)	22.8 (19.3–26.2)	23.0 (19.6–26.4)	23.9 (20.3–27.4)	0.42

↵^aAdjustment for age, BMI, menopausal status, age at menopause, age at menarche, smoking, and season of the year when mammography was conducted.
↵^bQuartile ranges: Q1, ≤25.04 ng/mgCr; Q2, >25.04 to ≤42.42 ng/mgCr; Q3, >42.42 to ≤67.52 ng/mgCr; and Q4, >67.52 ng/mgCr.

Additional Files

Tables

Supplementary Data

Files in this Data Supplement:

Supplementary Tables 1 - 3 - PDF file, 86KB, Supplemental table 1. Percent mammographic density and absolute density by selected characteristics of rotating night shift work by menopausal status. Supplemental table 2. Percent mammographic density and absolute density by selected characteristics of current rotating night shift work by menopausal status. Supplemental table 3. Adjusted means of percent density and absolute density according to the quartiles of urinary MT6s levels.