This is a very interesting phrase. The double vowel (diphthong) of “I” reduces to a short sound, as it’s weak. The real problem area, though, is the cluster of consonants /dntsl/. The solution is to remove the /t/ sound completely. It’s also important to keep the front of the tongue in contact with the top of the mouth all the time as you change from one consonant to another, otherwise, you’ll get a vowel where you don’t want one. This will take some practice. Listen and repeat using the recording. Do it slowly, especially at first.
This phrase is not an idiom. It simply means I was unable to get to sleep last night. I hope you sleep well!