Did Winston and Julia oversleep?

Question

When Winston and Julia are arrested at the end of Part 2 of 1984, Winston speculates dully that they may have overslept in the room above Charrington's shop, and that it is now the morning instead of the evening:

He noticed that he badly wanted to urinate, and felt a faint surprise, because he had done so only two or three hours ago. He noticed that the clock on the mantelpiece said nine, meaning twenty-one. But the light seemed too strong. Would not the light be fading at twenty-one hours on an August evening? He wondered whether after all he and Julia had mistaken the time — had slept the clock round and thought it was twenty-thirty when really it was nought eight-thirty on the following morning. But he did not pursue the thought further. It was not interesting.

_{-- Part 2, Chapter 10}

Later on, at the start of the next chapter, he is resigned to the fact that he will never know:

It might be twenty-four hours since he had eaten, it might be thirty-six. He still did not know, probably never would know, whether it had been morning or evening when they arrested him. Since he was arrested he had not been fed.

_{-- Part 3, Chapter 1}

Is there any further evidence for whether they were arrested in the morning or the evening?

Answer 1

Almost certainly yes.

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First of all, let's look at a few quotes from the previous chapter, just to set the scene:

Winston was gelatinous with fatigue. [...] He had worked more than ninety hours in five days.

Before he starts to read Goldstein's book together with Julia:

The clock’s hands said six, meaning eighteen. They had three or four hours ahead of them.

Then after he reads aloud more than five thousand words of text (which must take a while):

A yellow beam from the sinking sun slanted in through the window and fell across the pillow. He shut his eyes. The sun on his face and the girl’s smooth body touching his own gave him a strong, sleepy, confident feeling. He was safe, everything was all right. He fell asleep

So after a week of extremely hard work and sleep deprivation, followed by lovemaking with Julia and then (say) half an hour of reading, Winston falls asleep without setting an alarm clock. This must be at about 6:30pm, based on the same assumption of half an hour of reading.

_{From now on, all quotes are from Part 2, Chapter 10, and all emphasis is mine.}

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The very first sentence of Chapter 10 gives us our first hint at the truth:

When he woke it was with the sensation of having slept for a long time, but a glance at the old-fashioned clock told him that it was only twenty-thirty.

His glance at the clock tells us nothing: it's a twelve-hour clock, so it could just as easily be 8:30am or 8:30pm. The former would mean he'd had 14 hours of sleep; the latter, only 2 hours. His sensation of having slept for a long time already tips the probability in favour of the former.

‘I’m hungry,’ she said. ‘Let’s make some more coffee. Damn! The stove’s gone out and the water’s cold.’ She picked the stove up and shook it. ‘There’s no oil in it.’

‘We can get some from old Charrington, I expect.’

‘The funny thing is I made sure it was full. I’m going to put my clothes on,’ she added. ‘It seems to have got colder.’

All of this is even stronger evidence that they've slept much longer than they realised. Julia wakes up feeling hungry and significantly colder than before, but more importantly, the stove has completely run out of oil, despite being full before they fell asleep. How could that have happened in a mere 2 hours?

The sun must have gone down behind the houses; it was not shining into the yard any longer. The flagstones were wet as though they had just been washed, and he had the feeling that the sky had been washed too, so fresh and pale was the blue between the chimney-pots.

The sentence about the sun is inconclusive: either at 8:30pm (after the sun has set) or at 8:30am (when it's in a different part of the sky), it won't be shining in the same place as it was at 6:30pm. The wet flagstones suggest it's rained while they were asleep, which again is inconclusive: it could have started and stopped raining during a 2-hour period. But the sky is really damning evidence: at 8:30pm, more than an hour after sunset, the sky would be darkening. Fresh pale blue sounds like an early morning sky after rain.

Tirelessly the woman marched to and fro, corking and uncorking herself, singing and falling silent, and pegging out more diapers, and more and yet more.

Why would the prole woman be pegging out clothes in the evening, rather than taking them in?

He noticed that he badly wanted to urinate, and felt a faint surprise, because he had done so only two or three hours ago. He noticed that the clock on the mantelpiece said nine, meaning twenty-one. But the light seemed too strong. Would not the light be fading at twenty-one hours on an August evening?

This is when Winston finally realises what we, the readers, should have realised a few pages earlier. He needs the toilet again, suggesting a longer sleep than just a couple of hours, and (as mentioned above) it was too light outside to be 8:30 or 9 in the evening.

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As well as all the evidence from the text, as outlined above, it also makes sense from a plot-oriented point of view. We, and Winston, were never given any explanation as to why the Thought Police chose that particular moment to capture them. They already had them under surveillance, and could easily have waited months or years longer, if this was just another of their evenings spent in the hideout. As mentioned here, the longer they were 'free', the more likely they were to lead the Thought Police to other subversives. Arresting them would only be a sensible move if it somehow became necessary.

But if they'd accidentally slept the whole night in their hideout, then suddenly it becomes clear why they could no longer be allowed to remain at large in London. They would have been missed at home that night; once they realised their mistake, the Thought Police could no longer keep up the pretence of allowing them to think they were free while in reality watching their every move.

Oversleeping that night was their fatal mistake, and the reason they were finally arrested.

Answer 2

It's impossible to know. Orwell describes this entire situation with various statements that contradict each other, so it's not possible to get a definitive answer to this question.

In support that Winston was arrested in the morning:

and he had the feeling that the sky had been washed too, so fresh and pale was the blue between the chimney pots.

This suggests the sky as blue -- which only would've been possible in the day.

As he looked at the woman in her characteristic attitude, her thick arms reaching up for the line

This implies the woman is hanging clothes on a clothes line to dry them as the sun comes up. (This could instead suggest the woman is taking down the clothes, implying it's night.)

But the light seemed too strong.

This is another line suggesting that the light outside is too bright and illumines to be day.

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In support of him being arrested in the evening:

The sun must have gone down behind the houses; it was not shining into the yard any longer.

The sun sets during the night. (But this could also refer to the sun rising.)

"It seems to have got colder."

Generally, London is coolest during the nights, especially during the summer.

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Other pieces of evidence that are ambiguous:

When he woke it was with sensation of having slept for a long time, but a glance at the old-fashioned clock told him that it was only twenty-thirty.

As @Rand al'Thor pointed out, the fact that this clock was old-fashioned could mean he was arrested in either the morning or the evening, at either 8:30 AM or 8:30 PM. Winston has the feeling of sleeping for a long time, suggesting he could be arrested in the morning.

Ultimately, however, Orwell is vague and undescriptive as to what time of day Winston was arrested by providing bits of information that are ambiguous and can support both sides.