Rashi: When I lie on my bed I remember Your love.

Radak: [Also] when I remember You in my heart, on my bed, I am satiated. "Im" is in place of Ka'asher, like "v'Im Yihyeh ha'Yovel" (Bamidbar 36:4). "Yetzu'ai" is plural, for he did not lie in one place; he fled from place to place.

Mesilas Yesharim Perek 19: David describes his pleasure in speaking about and praising Hashem.

Malbim: Also if I remembered You on my bed, when I think about You at night, I remember how "You helped me" (verse 8, from enemies).

Rashi: It is at night.

Radak: They are the three times in the night when one wakes from sleep. There are three Ashmuros in the night (Brachos 3a). Verses mention "b'Ashmores ha'Boker" (Shemos 14:24) and "Rosh ha'Ashmores ha'Tichonah" (Shoftim 7:19).

Rashi: I think about You.