It turns out that the habit of staying up very late frequently, combined with the fact that teenage circadian rhythms are typically way out of balance lead to late night brain states inconducive for sleeping. It is unlikely that true insomnia has set in as a student enters college.
It is becoming so common for students to have problems sleeping, that it is required to take the definition of what is insomnia into question. Assuming this generally accepted criteria, a large amount of students would be consider insomniacs, however there is a difference in long run and short run insomnia. Insomnia set off by a sudden drastic change in habits (cramming multiple nights for a test)is called transient insomnia, it only lasts a few days and is primarily caused by increased stress/activity. Insomnia is usually thought of in the sense of acute or chronic insomnia. Acute lasts for a month or more, and chronic lasts over a long period of time uninterrupted.
Most insomnia can be avoided by maintaining a relatively steady sleep schedule. That said, some insomnia, particularly chronic can be caused by a chemical balance in the brain. These people usually have completely altered circadian rhythms as well. Subconscious cues like the level of light in the sky fail to set off the typical chemical messages. For some reason, in teens, circadian rhythms tend to differ in that they usually place subject at a disposition to wake up later and stay up later. This fact is observable throughout history, I remember in particular that in Romeo and Juliet, the aristocratic youth would always be awake wasting the night away very late, sometimes until the sunrise.
However, trying to balance this disposition to stay up later while maintaining obligations like early morning classes can lead to long term insomnia and many problems. In the image below, many effects of insomnia are displayed:
By Mikael Häggström (All used images are in public domain.) [Public domain], via Wikimedia Commons
