Which hormone is secreted more in dark conditions?

The pineal gland of the human brain secretes melatonin,which is concerned with:

Identify the correct gland on the basis of the following characteristics:
$(I)$ It is located on the dorsal side of the forebrain.
$(II)$ It plays a very important role in the regulation of the $24$-hour rhythm of our body.
$(III)$ It helps in maintaining body temperature.
$(IV)$ Its hormone influences metabolism as well as defense capability.

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

The diurnal rhythms are regulated by . . . . . . .

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