Maybe you stayed up late ringing in the new year, or changed time zones while traveling ... and dark to tell the body that it's day and night and coordinate those activities.

Experts say a spectrum of natural sleeping and waking rhythms exists, ranging from extreme morning people to extreme "night ...

They organized and assessed this data based on factors like time of day, species ... dawn and dusk), or cathemeral (active both day and night). Comparing the observational analysis with the ...