Table 2.

Mean differences between the first and last Insomnia Severity Index scores stratified according to work organization.

	N	First ISI–Last ISI			p value
		Mean difference	Confidence interval		Within	Between
All	1747	− 0.28	− 0.56	0.00	0.0473	–
Shift work experience
Never	4	− 2.00	− 6.68	2.68	< 0.0001	< 0.0001
< 5 years	1029	− 1.91	− 2.24	− 1.58
5–9 years	343	2.02	1.40	2.63
10–14 years	193	2.45	1.61	3.29
15–19 years	98	2.33	1.02	3.63
≥ 20 years	80	1.10	0.11	2.09
Quick return
Yes	522	− 0.64	− 1.13	− 0.15	0.0964	0.3003
No	1225	− 0.13	− 0.46	0.21
Consecutive night shift
1 day	259	0.58	0.01	1.15	0.0512	< 0.0001
2 days	202	0.10	− 0.69	0.90
3 days	1027	− 0.57	− 0.95	− 0.19
4 days	36	− 0.53	− 2.56	1.50
≥ 5 days	223	-0.28	-1.05	0.49
Nap opportunity
Yes	312	− 0.01	− 0.53	0.51	0.3716	< 0.0001
No	1435	− 0.34	− 0.66	− 0.02
Work-time control
Yes	486	0.15	− 0.31	0.60	0.0611	< 0.0001
No	1261	− 0.45	− 0.79	− 0.10
Weekly working hour
< 40 h	58	0.28	− 1.28	1.83	0.039	0.0001
40 h	345	0.41	− 0.22	1.04
41–51 h	765	− 0.30	− 0.73	0.13
52–59 h	243	− 1.35	− 2.16	− 0.54
≥ 60 h	336	− 0.27	− 0.81	0.27