axis also follows a circadian rhythm, with highest release within about 30 minutes to 1 hour upon awakening, called the cortisol response to awakening (CRA), and a slow taper throughout the day. Researchers have looked at both cortisol response to stress (CRS) and CRA to determine if attachment anxiety and avoidance underlie individual differences in HPA activity. Findings in the field have inconsistent. The evidence suggests that (a) individuals with high attachment anxiety perceive a higher degree of distress when faced with a stressor and have higher baseline anxiety compared to those low in attachment anxiety, (b) most studies suggest higher cortisol reactivity to stress in anxiously attached individuals, while two studies did not support this trend(c) avoidant-dismissive attachment has been less consistent, with some studies showing an increase in cortisol produced in response to a stressor, and others not finding any differences in comparison to securely attached individuals(d) fearful attachment is associated with lower cortisol both upon awakening and in response to a stressor although one study in pregnant women found less diurnal variation in those fearfully versus securely attached, leading to a higher bedtime cortisol level in the fearful group. The finding that those with fearful attachment would show small cortisol reactivity in response to a stressor may seem counterintuitive but is in line with the predictions of stress researcher Bruce McEwen, who hypothesized that frequent early stressors in life would cause an initial hyper-reactivity in the HPA axis that would over time become pathologically sluggish as the individual ages, leading to greater overall cortisol release and less adaptive responsivity. Another related biological system that regulates stress response is the autonomic nervous system. In general, the sympathetic system is activated during times of stress and the parasympathetic system acts to decrease physical readiness for stress. These systems are sometimes called the "fight-or-flight" and "rest-or-digest" systems, respectively, and operate in a balance, as opposed to being totally on or off. Researchers can approximate the relative balance by looking at skin conductance, blood pressure and heart rate. Studies of this kind provided hard evidence that avoidant infants were truly distressed during the strange situation task despite their minimally distressed appearance as they showed elevations in heart rate when caregivers were away and took longer to return to baseline when the caregiver returned in comparison to securely attached infants. Studies of skin conductance and heart rate in adults have shown that