I will admit, I borrowed the first part of the title from one of the student participants in the data you are about to see. As part of my upper level “Research in Human Biology” class, students collected saliva samples across two days – one self defined “stressful” day and one “less stressed” day. I deliberately did not define stress or less stressed days. This was, in my own way, a means of measuring their subjective experiences of stress. While the design of this class project came out well before the US News report, the timing could not have been better. My institution was ranked in the top 5 for undergraduate stress using a clears throat – flawed metric. It did provide however, a jumping off point for both this and the report we will have in a month. Stress was something students were already discussing – albeit a subjective measure. What the in class data collection promised to do was provide an objective, physiological measure of stress by measuring the amount of cortisol, a hormone involved in the stress response, in saliva samples.
Ideally, when measuring salivary cortisol, one would collect between 3 and 5 samples per day (Adam et al., 2010). However, in an effort to simply the collection protocols, we collected two samples – one at waking and one at bedtime. Again, samples were collected on two self defined days – stressed and less stressed. I had intended a “not stressed” day, but given current levels of student involvement in extracurricular activities, heavy course loads, and high expectations associated with each course, this became more of a fantasy on my part. I think faculty want to think that students exist only in an academic sphere – dorms are places of sleep, study, and deep discussion about how awesome their classes are, merely a bridge between academic endeavors. Having now collected 48 hours of activity data from these same students, I can assure you, they are busy. Tremendously, mind-boggling busy. It is quite impressive really.
The average waking cortisol sample for the class was 0.368 (0.166) ug/dL and the bedtime/pm sample mean was 0.274 (0.160) ug/mL. For self reported “less stressed” days, morning cortisol was 0.375 (0.177) and evening cortisol mean was 0.374 (0.177). Stressed days had an AM mean of 0.382 (0.22) ug/dL and an evening cortisol of 0.165 (0.146) ug/dL. To see that graphically, check out Figure 1, below.
So how do we interpret these findings? First off, I am not a stress researcher . . . I just play one in the classroom. My own passion, as the title of the blog suggests, is human milk. But, I did do some digging. What do these cortisol values mean (if you are looking at references with nmol/L, the conversion factor is the ug/dL times 27.59). Aardal and Holm (1995) give reference values of 14.6 nmol/L (0.56 ug/mL) as the mean for similarly aged females and 10.0 nmol/L for males in the morning (waking) and 1.1 nmol/L for females and 2.2 nmol/L for males in the evening (bedtime). If there are more recent articles with clearly defined stress cut-offs, I am happy to revise this post – but I did not find any on PubMed and calls via Twitter went unanswered. Compared to the Aardal and Holm reference values, physiologically, the students in my class are modestly but not severely stressed. Waking cortisol was 10.16 (4.57) nmol/L and bedtime was 4.97 (4.06) nmol/L (see above for values in ug/mL). Waking cortisol was slightly lower than average, whereas bedtime cortisol was high compared to the reference standards. This suggests that the students may have a blunted cortisol response.
This was true, and in a few students, salivary cortisol levels actually increased over the day (Figure 2) compared to the physiologically normal/ideal cortisol pattern of a decline from waking to evening (although the relationship is not quite that simple). Evening cortisol levels on stress days were often much lower than evening cortisol levels on less stressed days. The students chalked this up to “the stress being over” by bed, as their stress days were typically defined by exams, presentations, or interviews that were finished well before bedtime. Figure 3 shows a comparison of morning and waking cortisol levels by individual on stressed and less stressed days (colors are the same individual).
Figure 3a: Less stressed
Figure 3b: Stressed