Why do I wake up after just two hours sleep?

Many causes. I totally agree with dr. Villaplana. Depression can definitely do that. I also think of sleep apnea, alcohol effect (etoh is a sedative and helps you fall asleep but wakes you up in a few hours after it's metabolized, and due to it's diuretic effect), anxiety and menopause.
Osa. Dirrent reasons from obstructive sleep apnea to poor sleep hygiene--a lot of other causes like fibromyalgia, ptsd need more hx like how many night--how long stay awake.
? Depression. Early awakenings is one of the more specific symptoms of untreated depression. Having said that , without a thorough history, exam and labs as well as a very detailed sleep hygiene history, it's not possible to be accurate about the cause or causes for your symptoms. Please seek advice from a physician.
NREM to REM sleep. Every 90 minutes sleep pattern changes from non-dream sleep (nrem sleep) to dream sleep (rem sleep). The transistion from one sleep cycle to other and visa vers generally don't wake one up unless one is a light sleeper. Slow deep breathing exercise or pranayama in morning reguarly and prior to bed time may help to get to deeper sleep in may cases if one does over many months.