Then: Consciousness: what is it?  

Varying degrees of (biological) awareness/alertness, from hyper alert, through normal waking consciousness, through various altered states of consciousness, through the various levels of sleep to complete unconsciousness (as in a coma, where a person cannot be awakened). Varying degrees of consciousness can result from fatigue, illness, sleep deprivation, drugs and alcohol, focused attention, stimulus deprivation (or excessive stimulation), meditation, hypnosis, trance states, daydreaming, and the varying stages of sleep. Even states of unconsciousness can vary in terms of what and how much a person is aware of.

Biological rhythms:  Regularly occurring cycles of bodily changes tied to cycles of light/dark, heat/cold, etc. in the natural world and regulated by internal biological mechanisms referred to as 'biological clocks'.

Some such cycles are annual (hibernation in some animals, seasonal affective disorders in humans in far northern or southern regions of the earth), some monthly ( the human female's monthly menstrual cycles) and some are daily (circadian rhythms).

  Circadian Rhythms  The word circadian comes from the Greek for  'about' (circa)  'a day'(diem) Rhythms of variation in alertness/sleep, hormone levels, blood pressure, body temperature, amino acid levels, etc.)  Peaks of activity and metabolism cued by the bright light of day in humans (affects the pineal gland, the 'third eye' of the Greek myths)  Health,  sanity  and optimal performance are dependent upon respect for these rhythms: before the invention of electric lights, civilization's patterns and the body's needs were better matched.

The mechanisms for regulation of the biological clock's circadian rhythms are very complex;  different systems promote or inhibit sleep. The suprachiasmic nucleus of the hypothalamus, a tiny group of neurons, is located immediately above the optic chiasm (where the optic nerves cross to the opposite hemispheres); they stimulate the pineal gland in reaction to light. The pineal gland is 'turned on' to produce melatonin (which acts like a sedative) by darkness and dim light,  and 'turned off' by bright light, suppressing melatonin production . Connections through the hypothalamus to the endocrine system stimulate the secretion of such hormones as cortisol, which arouses bodily processes.  The Reticular Activating System is involved in stimulating the cortex and maintaining alertness when being awake/alert is called for. In addition, scientists have discovering a 'sleep hormone' that collects in the central nervous system during waking hours. (This hormone is not present in the bloodstream: conjoined (or 'Siamese') twins which share the same blood supply but have separate central nervous systems sleep at different times.) A person's circadian rhythms depends upon the synchonized  'see-sawing' of these opposing systems.

Our circadian rhythms runs at about 25 hours per day, not 24!  Each day, various time cues, such as daylight,  'reset' our internal clocks back an hour or so, but if we lived in an environment with no time cues, we would eat, sleep and be active on a 25 hour cycle... This explains why Monday mornings are so tough: we have to reset our 'clock' not by just one hour but for the extra time we did NOT 'reset' our internal clock, allowing ourselves to go to bed later and then get up later on the weekends. (More people have heart attacks on Monday mornings than any other time of the week!) 

The invention of artificial lighting and the imposition of clock-based (rather than daylight-based) rhythms on the body's natural circadian rhythms is negligible when the light is not too bright and the person is getting enough sleep, but  some of the other factors of modern living that disrupt our 'natural'  rhythms are: shift work, jet travel, and the bi-annual changing of the clocks for 'Daylight Savings Time'.  Ancient cultures did not demand we make such sudden time shifts. Due to the fact that our body's rhythms are set to a 25 hour day, it is always easier to adjust to sudden shifts to a longer or later-running day  than a shorter or earlier day. ( The adjustment is easier in traveling  from east to west rather than west to east, changing to a from day shift to evening to night, rather than the other way around. And it's an easier to get up an hour later in the fall than an hour earlier in the spring.... )