Table 4.

Night shift work characteristics of 59,947 Nightingale Study participants

Night shift work characteristic	N (%)
Ever/nevera
Never	11,799 (20%)
Ever	48,050 (80%)
Total number of years worked on night shifts b
1–4 years	5,695 (19%)
5–9 years	9,349 (31%)
10–14 years	6,103 (20%)
15–19 years	3,612 (12%)
20–24 years	2,668 (9%)
25–29 years	1,676 (6%)
≥30 years	1,395 (5%)
Cumulative lifetime number of nights worked b
<250 nights	6,082 (23%)
250–499 nights	6,026 (23%)
500–749 nights	4,567 (17%)
750–999 nights	2,977 (11%)
≥1000 nights	7,093 (27%)
Number of nights worked per month at cohort entry c
1 night per month	653 (7%)
2 nights per month	1,351 (14%)
3 nights per month	1,945 (21%)
4 nights per month	2,275 (24%)
5–7 nights per month	1,993 (20%)
>7 nights per month	1,223 (12%)
Number of consecutive nights worked at cohort entry c
1 night in a row	625 (7%)
2 nights in a row	2,307 (25%)
3 nights in a row	3,092 (33%)
4 nights in a row	2,209 (24%)
5 nights in a row	637 (7%)
6 nights in a row	209 (2%)
7 nights in a row	262 (3%)
>7 nights in a row	20 (<1%)
Type and direction of shift system at cohort entry c
Permanent	690 (7%)
Forward rotating	1,323 (14%)
Backward rotating	27 (<1%)
Variable	7,287 (77%)
Don’t know/missing	85 (1%)

^aEver night shift work was defined as having worked at least one hour between midnight and 5 am for at least one night per month.

^bAmong 31,265 participants who ever worked night shifts and provided detailed information.

^cAmong 9,889 participants who worked nights at cohort entry (i.e. in 2011).