We provide a definition of the unit internal (physiological) time based on metabolism. If q(t) is specific rate of metabolism, i.e. the amount of energy (oxygen) consumed by unit of active mass per physical time unit, the unit of physiological time τ(t) is defined as physical time, during which unit of active mass consumes one unit of energy: τ(t) = 1/q(t). The dimension of unit physiological time is the same as that of unit physical time and its value depends on q(t). Therefore, the unit physiological time τ(t) is a variable value, while the internal time is unequal relative to the physical time. The more internal time units τ, i.e., elementary acts of energy consumption, fit in the unit physical time t, the longer is the unit physical time for the unit active mass relative to the internal time unit, i.e., the physical time is seemingly slowed down. And, on the contrary, the less elementary acts of energy consumption take place during unit physical time, the shorter seems unit t, i.e. physical time is seemingly accelerated. Unequal course of the internal time is determined by the curve of specific metabolism q(t) during the life under specific conditions and, hence, internal time is individual. It has been questioned that the total (during lifetime) specific metabolism, often called Rubner constant, can serve as specie specific characteristic.