Can I sleep lying flat on days 0–6?

Not yet. Not yet. Sleep propped up at about 30 to 45 degrees for the first week to help limit swelling. A travel pillow stops you rolling onto the grafts.

Can I sleep lying flat on days 7–13?

Careful. Lying flatter is usually OK now. Still avoid sleeping face down or with the grafted area pressed into the pillow.

Can I sleep lying flat on from day 14?

Yes. Sleep however is comfortable.

Can I sleep lying flat after a hair transplant?

Usually from day 14, if your clinic agrees.

Day by day

Days 0–6Not yet
Not yet. Sleep propped up at about 30 to 45 degrees for the first week to help limit swelling. A travel pillow stops you rolling onto the grafts.
Days 7–13Careful
Lying flatter is usually OK now. Still avoid sleeping face down or with the grafted area pressed into the pillow.
From day 14Yes
Sleep however is comfortable.

Not yet reviewed by a doctor — verify with your clinic.

Sources

Hair Transplantation: Preventing Post-operative OedemaJournal of Cutaneous and Aesthetic Surgery, 2010
Complications in Hair TransplantationJournal of Cutaneous and Aesthetic Surgery, 2018
Hair transplantation surgeryIndian Journal of Plastic Surgery, 2008