Don't try to change the alarm clock setting during the middle of the night.
The other night my wife and I set our alarm and went to sleep. During the middle of the night I remembered that I would have to get up a bit earlier in the morning so, while still half asleep, I tried to update the alarm to be 30 minutes earlier.
We did not wake up 30 minutes earlier than planned, we work up nearly two hours later than planned. Why? Because in my sleepy haze I did not finish setting the alarm. When we woke up the timer was still blinking 8:45 AM, meaning that I did not turn the alarm back on.