George comes on a vacation to India from the $U.S.$ The long journey disturbs his biological system and he suffers from jet lag. What is the cause of his discomfort?

....... plays a very important role in the regulation of a $24$ hours rhythm of our body.

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

The organ which was considered vestigial until recently but is now confirmed to be an endocrine gland is:

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 hormone relates with temperature and sleep-wake cycle?