The biological clock that regulates circadian rhythm can be programmed and adjusted by the brain to help fulfill aspects of daily life, such as allowing a person to sleep early before an important event or wake up without an alarm. However, the biological clock is not easily altered without external influences when faced with environmental changes like jet lag or changing sleep locations. Circadian rhythm can be affected by external stimulants like drugs that alter neurotransmitters and hormones.

1. Circadian rhythm Can be programmed/adjust by our brain/plan.
The biological clock is not just merely influenced by environment (sun light ,temperature, seasons
, cities life etc..). But, seems to be also adjustable, in order to help us to fulfil some aspects of our
daily life.
For example:
If we have an important appointment/exam we will be able to sleep early ( However, in many
cases we are not easily able to sleep as a result of stress especially in something very important
eg: exam ) and what it is more we will find ourselves got wake up (may accompanied by
dreams/nightmares about that) in most cases without alarms early.
Besides in daily bases we wake up before our alarms ringing. In others words ,sleeping seems to
be regulated by the biological clock.
In addition, non environmental external stimulants can exist like: drugs.
But the environmental influences (i.e: jet lag , or changing sleeping place) seem are not easy to
be altered by our brain without an external influence like drugs.
This can be supported by drugs alter the circadian rhythms. or/and
The circadian rhythm can regulate the neurotransmitters and hormones (Adrenaline , Melatonin).