Plants have photochemical clocks that utilize photoreactive pigments such as cryptochrome to detect the length of the nighttime, allowing them to determine what time of year it is. They use this information to correctly time critical plant events like dormancy (summer for deserts, winter for temperate regions) and flowering. There is no point in wasting energy on flowering if other flowers and the proper pollinators aren't available!
Human life is largely constructed around the Circadian rhythm, which matches the 24 hour cycle of day and night. Light is the primary means by which the Circadian rhythm is kept in time with, or 'entrained', to the cycle of the Sun; other minor 'Zeitgebers' (from the German for 'time-givers') include temperature and patterns of eating and social interaction. Artificial lighting disturbs this normal cycle, as can disorders such as Delayed Sleep Phase Syndrome and Non-24-Hour Sleep Wake Syndrome.
When left to my own devices, I regularly go to bed between 3 and 5am. I often find myself wide awake long after everyone else is asleep. My peak work productivity usually falls between 10pm-3am, and I am almost completely nonfunctional before 10am. To force myself to be awake at the 'regular' times, I pull all-nighters at a frequency of about once per week so that I am awake in the morning, become exhausted, and can fall asleep at a 'normal' hour.
For these reasons, I've often wondered if I have either DSPS or Non-24. Many sleep disorder practitioners recommend keeping a sleep journal, but a more practical way to analyze activity patterns is to make use of activity data that you already have on-hand. For my analysis, I graphed several months of time-stamped text message and Google search records:
Though this is a graph of received text messages, texts are usually exchanged in volleys, so this should provide some quantitative sense of when I am awake. The text message data will tend to underestimate my probability of consciousness during the hours when no one in their right mind is awake (3-8am).
This data is much more useful, because it includes a much larger sample size and is not reliant on the participation of others.
The International Classification of Sleep Disorders diagnostic criteria for DSPS include:
1. ...a chronic or recurrent complaint of inability to fall asleep at a desired conventional clock time together with the inability to awaken at a desired and socially acceptable time.
5. Sleep-wake logs and/or actigraphy monitoring for at least two weeks document a consistent habitual pattern of sleep onsets, usually later than 2 a.m.
6. Occasional noncircadian days may occur (i.e., sleep is "skipped" for an entire day and night plus some portion of the following day)
7. The symptoms do not meet the criteria for any other sleep disorder causing inability to initiate sleep or excessive sleepiness.