Why do we feel sleepy when we are sick?

Hellooo Everybody,
Let's delve into some sleep physiology and learn about some sleep producing substances breifly!

The chemical agents which might be responsible​ for induction of sleep have been obtained from experiments on sleep-deprived animals.
The first experiments of this type were performed by Henri Pieron in 1913 on dogs. He demonstrated that dogs receiving CSF from sleep-deprived donor dogs slept for hours, while the recipients of CSF from normal donors remained awake. Recent work has confirmed these observations and identified several candidate sleep producing substances (SPS)

Sleep-deprivation presumably leads to a rise in the production of these substances in the brain tissue, cerebrospinal fluid,and even urine, through a negative feedback effect.

The best known SPS :
1) Muramyl dipeptide (MDP)
2) Delta sleep-inducing peptide (DSIP)
3) Arginine vasotocin (AVT)
4) Interleukin-1 (IL-1).

Besides these, there are about 20 more putative sleep-inducing factors.

Most of the known sleep producing substances induce Slow Wave Sleep.

After learning the physiology let's answer our question:
Muramyl dipeptide (MDP) is a component of bacterial cell( which acts via IL-1.) walls ,whereas IL-1 is released in infections.
IL-1 potentiates GABA-induced increase in permeability to chloride at synapses causing inhibitory effect on the Brain.

Nitric oxide may be part of a second messenger system which mediates the effect of IL-1 on sleep.

Lastly, IL-1 induces release of growth hormone releasing hormone (GHRH) which in turn releases growth hormone. Growth hormone itself enhances REM sleep and inhibits Slow wave sleep.

This explains why we feel sleepy when sick!

IL-1 also induces fever, and is therefore also called endogenous pyrogen.  Antipyretics, such as aspirin, suppress the fever induced by IL-1 but do not affect the sleep-inducing effect of IL-1.
Like fever, sleep is a smart response  which perhaps helps recovery by compelling the patient to take rest.

Isn't it truly remarkable, how our bodies work!

Let's Learn Together!
-Medha.