In 1 Nephi 1:6, we learned about Lehi's first visionary experience, which caused him to quake and tremble. Here, in 1 Nephi 1:7, we learn several things about the nature of that experience, and what it did to Lehi's physical body.
And it came to pass that he returned to his own house at Jerusalem; and he cast himself upon his bed, being aovercome with the Spirit and the things which he had seen.
It says he returned to his own house, which was at Jerusalem. This indicates that the experience that followed him praying with all his heart, did not occur in Lehi's home. It was elsewhere. The pillar of fire which appeared dwelt upon a rock. Perhaps it was without his home, just as Joseph Smith, in his first visionary experience, went to a grove of trees near his home. We don't know for sure with Lehi, but we do know that he returned home.
In returning, he cast himself upon his bed – his place of rest. I sometimes laugh when I read this line, because it makes me think of my kids when they have a tantrum, throwing themselves on their bed, kicking and screaming. I trust that's not what happened in Lehi's case. Often one's bed is one of the most energetically secure or sacred spaces in their home. There are a variety of reasons for this, but for myself, I enjoy reading and studying near my bed. For Lehi, he didn't go to the kitchen, the tv room, the busy areas. He went to his quiet space. And then he likely told himself, "Ok, breathe. You're not crazy. You just witnessed a pillar of fire. And more. Breathe..."
Being overcome with the Spirit, he cast himself upon his bed. There are several instances where other mortal men experienced this same symptom, of being overcome after experiencing something highly spiritual. In the book of Moses, it says that Moses experienced similarly:
9 And the presence of God withdrew from Moses, that his glory was not upon Moses; and Moses was left unto himself. And as he was left unto himself, he fell unto the earth.
10 And it came to pass that it was for the space of many hours before Moses did again receive his natural strength like unto man; and he said unto himself: Now, for this cause I know that man is nothing, which thing I never had supposed. (Moses 1:9-10, italics added)
It is apparently not a weakness of Lehi's, but something inherent in mortals that when they experience something otherworldly, their physical strength is often drained. This is reported to be the case with Sidney Rigdon, after he and Joseph Smith witnessed the vision(s) recorded in D&C 76. I suppose one explanation could be that God is full of light, much more than the sun at noonday. Just spending a few hours in the sun will tend to drain me. Perhaps spending even short amounts of time with beings in higher elements would tend to have this affect on a mortal.