Our cycles of wakefulness and sleepiness depend mainly on

A) changes in heart rate and blood pressure.
B) detection of changes in magnetic field.
C) hormones released by the adrenal gland.
D) cyclic activity generated within the brain itself.

Adrenal Gland

A gland located above the kidney that produces hormones such as adrenaline and cortisol, which help control stress, metabolism, and immune response.

Brain Itself

The organ located in the head of an organism, responsible for central nervous system functions, including processing sensory information and directing behavior.

Magnetic Field

An invisible field around magnetic materials or a moving electric charge, which exerts a force on other magnetic materials or moving charges.