Danny can't sleep:
Keep You Coming Slow by Wickedtruth.
If he finds it hard to sleep without this nightly ritual, it's just because he's a creature of habit, that's all. It's just a thing, a little thing. It doesn't mean anything at all. And maybe one day he'll believe himself. Explicit.
For A Little Peace Of Mind by Gehirnstuerm.
Danny is bone-deep exhausted, but he can't sleep. Creative measures have to be taken. Rated Gen.
To sleep, perchance to dream by Sirona.
The one where Danny can't sleep without Steve. It's Grace's birthday, and all she wants is for the Five-0 team to take her camping. Cliche showcased: enforced bed sharing. Rated Gen.
And a delightful snippet of sleepy!boys:
Blaming the sun by Ellievolia.
Steve and Danny have finally the chance to catch on some sleep. Rated Gen.