In many organisms on this planet Earth, there is a biochemical cycle called the Circadian rhythm. Now, this cycle is something that is typically conditioned to suit the external environment. This is generally done by the retina in your eyeball, which have photosensitive receptors (rods and cones) that help form the routine of your Circadian rhythm.
Depending on how much daylight there is, your body will begin to routinely take certain actions such as hormone secretion and temperature regulation. More notably, in this context, is the creation and release of Melatonin during the lack of sunlight.
When it begins to get dark outside, the pineal gland in your brain begins to secrete Melatonin and this primarily induces feelings of drowsiness. For those with insomnia, one of the first things that will be prescribed will be Melatonin, an over-the-counter product.
Now what does this all have to do with sleeping at night?
Well first of all, you're going to have to resist the effects of Melatonin all night. Is it really worth literally resisting your body's call for sleep?
Secondly, you will slowly be affecting your Circadian rhythm. It takes time to recondition your original pattern to suit the new cycle. With school starting again in just over a weeks time, you'll find it very difficult to focus and even stay awake unless you correct your sleeping routine right now.
Sleep also gives time for your brain to focus on hard-wiring facts into memory, preferably long-term. This is why it's recommended for students to study right before sleeping because that information is the most fresh. This is contrary to the belief that studying in the morning is the most ideal due to feeling rejuvenated.
There is also a concept called Sleep Debt, where a slight lack of sleep each day begins to accumulate and once the "debt" is large enough you will begin to notice increased physical and cognitive fatigue.
What else, studies show proper sleep is connected with immune function and human life spans. These are both positive correlations.
I dunno I tried. I'm a bit rusty since I'm no longer majoring in Biology but hopefully this was an interesting read. Also, Physics is evil.