Ii had a dsicussion with some friends about this and been thinking alot about it. My question is, why do we sometimes laugh easily/alot when we are tired/sleepy from a strict scientific view?

This slight euforic feeling that makes alot you talk about laughable which almost resembles a "high".

People often make the comparison between sleep deprivation and the effects of alcohol as both tend to depress areas of the prefrontal cortex. The prefrontal cortex is an area of the brain which is invoved rational thinking, judgment, emotion etc so being tired is a lot like being drunk. As a result, when tired you're generally more relaxed, have heightened emotions and I find tend to be more silly which ineffably leads to giggles.

Also if you're very tired, you don't really care about your actions as much. You get quite lazy. So things I don't mean to say to people sometimes slip out when I haven't had enough sleep, which has gotten me in trouble at work more than once.

It also depends on the time of day/night. There are diurnal effects caused by changes in Hypothalamic-Pituitary Axis (HPA) activity and hormone levels (melatonin, cortisol etc.).

 

For example, if you stay awake all night, the lowest part of your 'day' will be between 2 and 4 in the morning. When morning arrives, melatonin levels drop and cortisol levels spike, waking your body up. This leads to feelings of energy and euphoria.

 

If you do this regularly (e.g. working night shift) your body adapts after about 5 days and the effect goes away.