He didn't get the alert until it was far too late. Everyone in the Mexican camp was dead tired from the weeks of force-marching.
The other thing that you should consider is that the previous day, he'd force-marched his troops all day, set up camp for the night, discovered that the Texians weren't far away, and had his troops erect hasty fortifications and stand-to to repel a night attack.
The night attack never came, his troops were exhausted, they had to rest. He posted additional sentries, figured that a "peasant rabble" could never successfully attack his force during the day, and took a nap.
And Sam Houston, the failure of failures, who had been accused of running away from everything and everyone, brought his outnumbered but enraged troops in and crushed Santa Anna's army in 18 minutes.
Well, it was 4 PM, so the Mexicans would've been "napping" whether they were tired or not. That's siesta. Santa Anna was otherwise occupied, I'm sure you've heard.
Miss Emily West, liege to Col. James Morgan (so, sometimes identified as Emily Morgan) of Morgan's Point, was entertaining el jefe en flagranto y con opium, which helped hasten his surrender.
Miss Emily, who was of mixed blood, is known to many as The Yellow Rose of Texas, and was the sweetest little rosebud that Texas ever grew (though she was from New Haven, CT, possibly Jamaica before that). A heroic Texian.