Learn Russian: Под утро наше наблюдение показало новую фазу сна. - Towards morning, our observation showed a new sleep phase.

While к утру (with the Dative case) means 'by morning' in the sense of a timeline or deadline, под утро (using под + Accusative) specifically means 'just before morning' or 'approaching morning.' It emphasizes the time being right on the edge of morning.

Why does под take the Accusative case here (утро) instead of the Instrumental case?

The preposition под takes the Instrumental case when talking about physical location ('under'). However, when we use it to express an approximation of time or age (meaning 'getting close to'), it always requires the Accusative case. Since утро is a neuter noun, its Accusative form looks identical to its Nominative form.

The observation happened over a long period, so why is показало in the perfective aspect rather than the imperfective показывало?

Although the observation itself was an ongoing process, the verb describes the specific result it produced. The perfective показало is used because it focuses on the single, successfully completed event of revealing the new sleep phase, not the ongoing process of watching.

Where did the 'о' go in the word сна?

The dictionary form for 'sleep' is сон. When it changes to the Genitive case here to mean 'of sleep' (фазу сна), the о drops out. This is a classic example of a 'fleeting vowel' in Russian, where an 'о' or 'е' disappears from the root when grammatical endings are added.