The salivary cortisol awakening response is thought to
be a feature of basal hypothalamic–pituitary–adrenal (HPA) axis function. This
profound increase of cortisol after awakening might be influenced by sunlight.
Another, less well-studied salivary biomarker showing a diurnal cycle is
salivary alpha-amylase (sAA). Although sAA is often proposed as a marker for
stress-related changes of sympathetic activity, only little is known about profound
changes after awakening and possible confounders. The present study examined
the effect of sunlight (waking up in the dark vs. waking up with daylight) on
cortisol and sAA awakening responses in a natural setting, and whether
responses vary with gender.
Data were collected from 6 men and 16 women aged 18 to
55 years. Saliva samples were collected immediately after waking up and 30
minutes later on 2 consecutive days (dark vs. bright). Compliance with the
sampling protocol was assured with the help of electronic monitoring devices.
Data on sleep quantity, time of awakening, and mood were also obtained.
Neither cortisol nor sAA levels on waking
differed by gender, or between dark and bright day. However, the cortisol
awakening response differed between dark and bright day (p=0.013), with higher
responses on the bright day. On either day, women showed larger cortisol
increases than men (all p<0.05). We found no effect of sunlight or gender on
sAA responses. Time of waking up, mood, and sleep quantity were not related to
the cortisol and sAA awaking response on either day.
Our results indicate that
the cortisol awakening response is at least partly influenced by sunlight while
there was no effect of sunlight on amylase awakening responses. The
suprachiasmatic nucleus (SCN), the endogenous biological clock, is known to be
sensitive to light signals from retinal ganglia cells and to regulate basal HPA
axis activity. Despite there are autonomic projections of the SCN to different
peripheral tissues, basal alpha-amylase activity seems to be regulated via
different pathways.
