Endocrine (cortisol) variables

Figure 3.01 shows mean cortisol values at six collection times over 24 hours for shift-workers on day shift and night shift, and control participants on day shift only. Visual inspection shows that control participants and shift workers on either day shift or night shift had broadly normal cortisol rhythms, with a falling off of cortisol level over the evening as the normal sleep time approached, and a late night or early morning quiescent period followed by a morning peak.

58 0 2 4 6 8 10 12 14 16 1500h 1900h 2300h 0300h 0700h 1100h Tim e of day C or ti s ol n m ol /L

Figure 3.01. Mean cortisol levels (nmol/L) at the six sampling times over 24 hours for shift workers on day shift (dashed), shift workers on night shift (dotted) and control participants (solid). Errors bars represent the standard error of the mean.

3.2.3.1. Control day shift vs shift worker day shift cortisol

Figure 3.01 shows that a sinusoidal rhythm was apparent for both control participants and shift workers regardless of shift worked. For shift workers, the cortisol levels at the acrophase of the rhythm were higher for day shift than for night shift. Control participants and shift workers had similar levels of cortisol at 1500h and 1900h during a day shift. At 2300h, control participants showed an atypical rise compared to shift workers, whose levels appear to be following the normal waning into the quiescent period. At 0300h, shift workers showed an early rise in cortisol level compared to control participants, who still appeared to be in the overnight quiescent period of the rhythm. By 0700h control cortisol levels had peaked, along with shift workers, while both groups showed a normal falling off of cortisol as the morning progressed (1100h).

Independent samples t-test did not reveal a significant difference between shift workers and control participants for mean 24-hour cortisol levels over a day shift, t(25) = 0.22, p > .05, r = .04, or for total 24 hour cortisol levels, t(25) =

0.34, p > .05, r = .07 (see Table 3.03). Shift workers did not have significantly

higher day shift cortisol levels than control participants at any of the collection times over a 24-hour period (for all comparison, p > .05 with small effect sizes,

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see Table 3.04). Although cortisol levels at 2300h did not significantly differ between shift workers and control participants, a medium effect size was found,

t(23) = 1.79, p > .05, r = .38.

Table 3.04: Independent t-test and Kolmogorov-Smirnov Z test results for difference between cortisol levels of control participants and shift-workers on day shift

Shift workers Control participants

Collection time M SD M SD t D p r 1500h day shift** 5.80 4.78 5.48 2.80 0.23 - .83 .04 1900h day shift## 2.00 1.89 2.60 1.19 -0.89 - .39 .23 2300h day shift# 1.44 1.42 3.76 4.81 -1.79 - .09 .38 0300h day shift 7.22 8.54 2.59 1.69 - 0.44 .10 .21 0700h day shift* 11.70 6.27 12.86 6.46 -0.46 - .65 .09 1100h day shift† 7.56 6.27 6.39 2.92 - 0.22 .73 .10 24h dayshift total* 34.10 13.57 32.52 10.51 0.34 - .74 .07 24h dayshift mean* 6.03 2.27 5.84 2.01 0.22 - .83 .04

* = shift workers n = 10, controls n = 17; ** = shift workers n = 10, controls n = 16; # = shift workers n = 9, controls n = 16; ## = shift workers n = 10, controls n = 14; † = shift workers n = 12, controls n = 17

3.2.3.2. Control day shift vs shift worker night shift cortisol Figure 3.01 shows that on average, cortisol levels at 1500h and 1900h did not differ between control participants on day shift and shift workers on night shift. At 2300h, control participants showed a slight elevation compared to shift workers; however by 0300h shift workers on night shift had slightly higher cortisol. At 0700h, control participants showed a higher level. By 1100h both groups’ cortisol had fallen off, although shift workers were lower on average. Over a 24-hour period, there was not a significant difference between the mean,

D (10,17) = .41, p > .10, r = .20, or total, D(10,17) = .39, p > .10, r = .19,

cortisol levels of control participants on day shift and shift workers on night shift, (see Table 3.05). Cortisol levels did not differ significantly at 1500h t(24)

= 0.48, p = .64, r = .10, nor at 1900h, t(21) = 1.82, p = .10, although there was

a large effect size, r = .52. At 2300h, 0300h, 0700h and 1100h, control

participants and shift workers did not differ in cortisol levels (all p > .05), and

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Table 3.05: Independent samples t-test and Kolmogorov-Smirnov Z test results for differences between cortisol levels of shift-workers on night shift and control participants on day shift

Shift workers Control participants

Collection time M SD M SD t D p r 1500h* 6.10 3.93 5.48 2.80 0.48 - .64 .10 1900h# 5.00 3.84 2.60 1.19 1.82 - .10 .52 2300h** 1.67 1.58 3.76 4.81 - 0.37 .20 .18 0300h* 4.10 6.76 2.59 1.69 - 0.24 .69 .12 0700h## 8.13 5.62 12.86 6.46 - 0.45 .11 .21 1100h## 4.63 2.33 6.39 2.92 - 0.38 .21 .19 24 hour total† 26.40 11.25 32.52 10.51 - 0.39 .18 .19 24 hour mean† 4.82 1.91 5.84 2.01 - 0.41 .17 .20

* = shift workers n = 10, controls n = 16; ** = shift workers n = 9, controls n = 16; # = shift workers n = 9, controls n = 14; ## = shift workers n = 8, controls n = 17;

† = shift workers n =10, controls n = 17

3.2.3.3. Night shift vs day shift cortisol levels for shift workers Figure 3.01 shows that on average, shift workers on a night shift had higher mid-evening (1900h) cortisol levels, a relatively slower morning rise of cortisol and a lower acrophase (0700h) compared to the rhythm seen on a day shift. Paired samples t-tests and the Wilcoxon Signed Ranks test were used to compare cortisol on a night shift versus on a day shift (see Table 3.06). On average, mean cortisol levels over 24 hours were higher during the day shift compared to the night shift. Although this difference was not significant, a large effect size was found: t(9) = 1.91, p > .05, r = .54. However, total cortisol over

a day shift was significantly higher than that seen over a night shift, and the effect was large, t(9) = 2.37, p < .05, r = .62.

At 1500h, there was a negligible difference between day shift and night shift cortisol levels, t(9) = 0.18, p > .50, r = .06. At 1900h, although the difference

between day shift cortisol and night shift cortisol was not significant t(8) =

2.19, p > .05, there was a large effect size, r = .61. There was no significant

difference between day shift and night shift cortisol levels at 2300h, t(7) = 0.16, p > .5, r = .06.

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Table 3.06: Paired samples t-test and Wilcoxon signed ranks test results for differences between shift workers’ day shift and night shift salivary cortisol levels

Shift workers Day shift Night shift

Collection time M SD M SD t T p r 1500h (n = 10) 5.80 4.78 6.10 3.93 -0.18 - .86 .06 1900h (n = 9) 2.00 1.89 5.00 3.84 -2.19 - 06 .61 2300h (n = 8) 1.44 1.42 1.67 1.58 -0.16 - .88 .06 0300h (n = 9) 7.22 8.54 4.10 6.76 - 10.50 .19 -.34 0700h (n = 8) 11.70 6.27 8.13 5.62 1.01 - .35 .36 1100h (n = 8) 7.56 6.27 4.63 2.33 1.27 - .25 .43 24 hr total (n = 10) 34.10 13.57 26.40 11.25 2.37 - .04 .62 24 hr mean (n = 10) 6.03 2.27 4.82 1.91 1.91 - .09 .54

For the morning collection times (0300h, 0700h and 1100h), on average cortisol was higher during the day shift compared to night shift levels (see Figure 3.01). Although these differences were not significant, medium to large effect sizes were evident: at 0300h T(9) = 10.50, p > .05, r = -.34; at 0700h t(7) = 1.01, p =

.35, r = .36; and at 1100h t(7) = 1.27, p = .25, r = .43.

3.2.3.4. Nocturnal cortisol levels

Nocturnal cortisol levels were calculated by summing 2300h and 0300h levels for shift workers on both day shift and night shift, and control participants on day shift (see Table 3.07).

Table 3.07: Kolmogorov-Smirov Z test and Wilcoxon Signed Ranks test results for differences between shift workers’ and control participants’ total (panel B) and mean (panel C) nocturnal cortisol (2300h+0300h)

Total Mean A: 2300h+0300h M SD M SD

Shift workers (SW) day 8.67 8.53 4.33 4.26 Shift workers (SW) night 3.67 2.12 3.95 6.77

Control participants (C) day 5.83 4.58 3.40 3.31 B: Total 2300h+0300h D T p r SW day vs SW night - 6.50 0.27 -0.32 SW day vs C day 0.25 - 0.27 0.10 C day vs SW night 0.22 - 0.96 0.11 C: Mean 2300h+0300h D T p r SW day vs SW night - 14.50 0.68 -0.12 SW day vs C day 0.21 - 0.68 0.12 C day vs SW night 0.15 - 0.76 0.07

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The Wilcoxon Signed Ranks test and the Kolmogorov-Smirnov Z test were used to compare groups. Nocturnal cortisol did not significantly differ between shift workers on day shift and control participants on day shift for either mean,

D(9,16) = 0.25, p (two-tailed) > .50, r = .12, or total levels, D(9,14) = 0.21, p

(two-tailed) > .50, r = .10.

There were no significant differences between either mean, T = 14.50, p (two

tailed) > .50, r = -.11, or total, T = 6.50, p (two tailed) > .10, r = -.32, nocturnal

cortisol on a day shift and on a night shift for shift workers, although the difference for total nocturnal cortisol represented a medium effect size.

Nocturnal cortisol of Control participants on a day shift did not significantly differ from that of shift workers on a night shift for either mean, D(10,17) =

0.15, p (two-tailed) > .50, r = .10, or total levels, D(9,15) = 0.22, p (two-tailed)

> .50, r = .11.

3.2.4. MRI volumetric variables