The $24$ hour (diurnal) rhythm of our body such as the sleep wake cycle is regulated by the hormone

Steroid hormone is derived from

Somatostatin from hypothalamus gland

Pancreatic Somatostatin

Mammalian thymus is mainly concerned with

$I.$ Sleep-wake cycle   $II$. Body temperature   $III$. Pigmentation    $IV$. Metabolism   $V$. Defence capability

All of the above written activities are influenced/regulated by