Why don't we feel hungry after waking up?
I will try to keep the answer as simple as possible, but feel free to point out if I make this too simple.

Well, you are not alone with this observation. This is quite a common, but rather complex, phenomenon which controls our apetite and causes us to not feel hungry after waking up.

The feeling of being hungry is under constant regulation by two antagonistic (having opposite effects) hormones:


leptin, released from adipose tissue, decreases our apetite and suppresses our hunger
ghrelin, released from stomach, does just the opoosite i.e. increases our hunger.


And guess what, these hormones are controlled through the circadian cycle. When you sleep, your body produces more leptin and stops the production of ghrelin. Why? The answer is quite obvious: because you're sleeping. While sleeping, your metabolism is lowered because your body needs to repair the damaged cells and tissues. To control metabolism, we need to control our apetite, and to control our apetite, we need to suppress our hunger. So...? When you wake up, you still have a hangover of this effect. In other words, it takes some time to change the secretion rates of leptin and ghrelin and the resulting effects.

Lets get into some analytical details now. According to Sharma et al, 2010, metabolic rate and brain temperature are lowered during non-REM sleep to deal with the damages that may have occured during time of wakefulness. In their study, the subjects were deprived of sleep for 2 nights (4 hours per night) and then allowed to sleep for next 2 nights (10 hours per night). The result? Leptin levels decreased by 18% and ghrelin levels increased by 28%. The subjects also craved for more carbs. Also, craving for salty foods increased by 45%. Overall hunger increased by 23% due to reduction in leptin levels. They also showed that people who sleep less than 5 hours have increased Body Mass Index (BMI). All this points to the fact that sleep does cause reduction in apetite and hunger. In short, don't think of missing your breakfast.

References:


Sharma, Sunil; Kavuru, Mani (2 August 2010). "Sleep and Metabolism: Overview". International Journal of Endocrinology. 2010: 1–12. doi:10.1155/2010/270832.
Wikipedia contributors. "Sleep and metabolism." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 8 Feb. 2017. Web. 26 May. 2017.