I thought about you all night.
o pen-SA-to a te TUT-ta la NOT-te — stress on 'sa-', 'te', 'tut-', 'not-'. Double 't' in 'notte' gives a slightly clipped sound.
Said the day after a meeting or date — confessing that they occupied your thoughts through the night is a genuine and romantic admission.
'Ho pensato a te' = I thought about you (passato prossimo of 'pensare'). 'Tutta la notte' = all night. The admission of sleeplessness or night-long thinking is a classic Italian romantic motif — the inability to sleep because of someone.
Non riuscivo a dormire — non facevo che pensare a te.
I couldn't sleep — I couldn't stop thinking about you.
More specific — names the sleeplessness and its cause
Ti ho sognato.
I dreamed about you.
Intimate — the unconscious mind reveals the depth of the attraction
Sei stata/o l'ultima cosa a cui ho pensato prima di addormentarmi.
You were the last thing I thought about before falling asleep.
Tender — ending the day with them in mind is deeply romantic
The wakeful night ('notte insonne') caused by romantic thoughts is a staple of Italian poetry from Petrarch to the present. Telling someone 'ho pensato a te tutta la notte' invokes this tradition — it is romantic hyperbole that is taken at face value and appreciated as sincere expression.