Why is it so hard to wake up to a normal alarm clock?

Because a normal alarm clock can't detect where you are in your sleep cycle - a continuous cycle from deep sleep, to brief almost-awake moments, and back to deep sleep again. Occasionally, your alarm may catch you at an optimal, almost-awake moment and you wake up feeling refreshed, but usually you grope for the snooze button waking up tired and groggy.

What is an "almost-awake" moment?

Almost-awake moments occur throughout a night's sleep. Sometimes an almost-awake moment is triggered by an outside influence like a loud noise or a dog jumping on the bed, but usually these moments happen around REM sleep. When you are almost awake, you may move around, or sit up to look at the clock, but then you usually drift quickly back to sleep. You probably won't even remember having been awake. Almost-awake moments are very brief, usually lasting less than 20 seconds. 

What can influence your sleep?

No two people have the same sleep cycles, and nobody has the same cycle twice. Many factors can influnce sleep cycles, including diet, exercise, medications, drugs or alcohol, stress, sleep disorders, and sleep deprivation. Age and gender can play a role, too: women tend to sleep more soundly than men, and as we age, we sleep more restlessly.