How do you look at and use the word: alertness?

Especially regarding the general function of waking up at about 6:00 am
in the morning following a night's sleep of say eight hours?

Regarding subtle brain injuries involving the brain stem of the brain,
how have you seen the word/term alertness used?

Have you noticed that alertness can vary enormously during the day?
Have you noticed certain times of the day (which likely normally vary
from person to person) which tend to seem to be personal alertness
times (windows of good alertness time) which can last for an hour or so
vs other hours of the day where alertness levels seem be noticeably
lower?

Your insights into the use of the term alertness please.