Which of the following hormones helps in maintaining the normal rhythms of the sleep-wake cycle and body temperature?

$I.$ Sleep-wake cycle, $II.$ Body temperature, $III.$ Pigmentation, $IV.$ Metabolism, $V.$ Defense capability. All of the above-written activities are influenced/regulated by:

Daily rhythms are usually associated with

The recently discovered hormones, melatonin and serotonin, are secreted at the ends of nerve fibres by the activity of:

Which gland plays a very important role in the regulation of the $24$-hour (diurnal) rhythm of our body?

Melatonin is secreted by the . . . . . . .