in vs for ( regarding time )

Note: I'd leave out the "a". Alternatively, you could also say "I've not slept well..."

I'd say "in the past few days" indicates the condition of poor sleep is on again off again. That is, there are days where you sleep okay and there are days where you sleep poorly.

I'd say "for the past few days" indicates that the condition of poor sleep in continuous or largely so. At no time during that stretch did you experience good sleep.

I would say:

I haven't slept well for days.

I haven't had a salubrious sleep for the past few days.

salubrious

• favorable to or promoting health; healthful: