The Psalms were sung at night long before they were printed. Several were written by a man who could not sleep, and at least one was written for the priests who kept the night watch in the temple. They are the Bible's own bedtime book.
Rather than one answer, here are seven, each matched to a kind of night. Pick the one that names yours. Each links to the full chapter with the audio, so you can read the first verse and let the rest be read to you.
Seven psalms, seven nights
8 I will both lay me down in peace, and sleep: for thou, LORD, only makest me dwell in safety.
For an ordinary restless night. The whole psalm is eight verses and it ends in bed.
1 He that dwelleth in the secret place of the most High shall abide under the shadow of the Almighty. 2 I will say of the LORD, He is my refuge and my fortress: my God; in him will I trust.
For fear. This is the psalm people leave playing all night, and the one carried through wars and hospital nights for centuries. The terror by night is named in verse 5 and dismissed.
1 [A Psalm of David.] The LORD is my shepherd; I shall not want. 2 He maketh me to lie down in green pastures: he leadeth me beside the still waters. 3 He restoreth my soul: he leadeth me in the paths of righteousness for his name's sake. 4 Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me.
For grief. The valley of the shadow of death is walked through, not around, and the shepherd walks it too.
1 [A Song of degrees of David.] LORD, my heart is not haughty, nor mine eyes lofty: neither do I exercise myself in great matters, or in things too high for me. 2 Surely I have behaved and quieted myself, as a child that is weaned of his mother: my soul is even as a weaned child.
For an overactive mind. Three verses. A weaned child with its mother. Nothing to figure out.
10 Create in me a clean heart, O God; and renew a right spirit within me. 11 Cast me not away from thy presence; and take not thy holy spirit from me. 12 Restore unto me the joy of thy salvation; and uphold me with thy free spirit.
For guilt. If what keeps you up is something you did, David wrote this after the worst thing he did, and he slept again.
1 [A Song of degrees for Solomon.] Except the LORD build the house, they labour in vain that build it: except the LORD keep the city, the watchman waketh but in vain. 2 It is vain for you to rise up early, to sit up late, to eat the bread of sorrows: for so he giveth his beloved sleep.
For exhaustion and overwork. Sitting up late to eat the bread of sorrows is called vain, and sleep is called a gift.
1 [A Song of degrees.] Behold, bless ye the LORD, all ye servants of the LORD, which by night stand in the house of the LORD. 2 Lift up your hands in the sanctuary, and bless the LORD.
For quiet. The shortest song of ascents, addressed to those who stand by night in the house of the Lord. Say it as your own blessing and turn out the light.
If you want one to memorize, make it Psalm 4:8. If you want one to play, make it Psalm 91. If you want to stop deciding, put on the livestream and the whole book will come to you in order.
Questions
What is the best psalm for sleep?
Psalm 4 for an ordinary restless night, Psalm 91 for fear, Psalm 23 for grief. Psalm 91 is the one most people leave playing all night.
Is Psalm 91 a bedtime psalm?
It has been used that way for centuries. Verse 5 names "the terror by night" directly, and the psalm's promise of shelter is why it is read at bedtime and over the sick.
How long should I read before bed?
One psalm. The point is to end the day on scripture, not to finish a chapter count. If your eyes are tired, use the audio.