Two indoleamines are secreted from rat pineal gland at night and act on melatonin receptors but are not night hormones.

Blood levels of NAS and NAT were insufficient to activate peripheral melatonin receptors, and they were invariant, so they could not serve as circulating hormones of night. However, they could instead act in paracrine circadian fashion near the pineal gland or via other higher-affinity receptors.