A/N: This story is set somewhere between Forests of the Dead and Turn Left, possibly after Midnight. I think the Doctor's reactions to Midnight have been explored excellently in many other fics, so I went off on a different tangent.
It's never really night or day in the TARDIS. When you're tired, you sleep, and when you're bored, you land somewhere and get into trouble. If you're a Time Lord and have a human on board, you while away the hours that they waste on a circadian rhythm by twiddling around and tweaking various bits of machinery. If you happen to be the human, though, all the hopping between planets and eras can really throw your sleeping patterns into disarray.
Right now, at what her mobile told her was half past two in the morning, Donna had been trying to get to sleep for several hours. She finally gave up, put her fuzzy robe on and went off to look for the Doctor. Maybe he would like to join her for a cup of tea. When she got to the console room she found it empty, but just as she was about to head off to the kitchen on her own, she heard a small sound coming from the odd-looking sofa by the console. Coming around the nearby pillar, she saw the Doctor curled up on the sofa, fast asleep. She was a bit surprised that he could fit on there; he usually took up so much more space than his gangly frame would require, and Donna had never seen him looking so small. He appeared to be dreaming, eyes moving erratically behind their lids. As Donna watched, he winced and started mumbling in his sleep. She didn't understand a word – was he speaking Gallifreyan? Why didn't it translate? Whatever it was, it was clearly a very bad dream. The Doctor's trainer-clad feet twitched where they poked out over the edge of the sofa, and his voice was full of anguish. In fact, Donna was glad that she didn't understand him, that she didn't get that insight into his mind. It made her heart ache just seeing him this distressed.
For a second, she contemplated waking him, but she quickly dismissed the idea. She knew how private he was; he wouldn't want her to have seen him like this. So instead she retreated, backing off the way she had come while wishing she could do something to comfort him and banish the guilt that this short episode had left her with.
She was almost into the corridor when there came a clattering thud from behind her, making her jump and turn around. The Doctor had fallen off his makeshift bed and was now manoeuvring himself into a sitting position. He hadn't noticed her presence, and she could see the ravages of the nightmare still lingering on his features. He would see her as soon as she moved, so she started walking towards him very deliberately before stopping dead and exclaiming 'What are you doing on the floor?' in her best you-weird-time-boy voice.
The Doctor looked up at her, a split-second of shock in his eyes before he turned away, scrubbed his hands down his face and picked something up off the floor. The next moment, he had jumped to his feet with his usual grin slapped on.
'Dropped my sonic!' he said, waggling it back and forth between thumb and forefinger. She rolled her eyes at him, and he continued, 'And what are you doing awake at this hour?'
Donna didn't bother asking how he knew what hour that was without looking at a clock. He always knew.
'Couldn't sleep, could I? The way we keep jumping about, it's like a jet-lag on steroids! Thought I'd get some tea or something. Jammie dodger?' She cocked her head at him. 'How's that sound?'
'Sure, great!' the Doctor enthused, pocketing his screwdriver and trooping after Donna to the kitchen. She set about making tea, avoiding looking at him. She didn't often feel this awkward around him, and she was sure that her guilt and confusion would show if he saw her face. However, the Doctor had plopped himself in a chair and seemed utterly oblivious to her mood, his cheery demeanour from earlier having fallen away like a dropped cloak. Donna observed him out of the corner of her eye as she rummaged through a cupboard for the biscuit tin. He looked exhausted and more than a little lost where he sat with his head in one hand, fiddling absent-mindedly with the TARDIS-shaped salt shaker.
Setting the biscuits down in front of him, she remarked with what she hoped was her usual chipper bluntness. 'You look rather terrible, do you know that? Maybe you should just get to bed and I can look after the TARDIS for the night.'
'Nah. Couldn't sleep right now anyway. Just some tea, thanks, that'll sort me out – does wonder for the synapses.'
'Yeah, well, you've got to sleep at some point, I'm sure. Even Time Lords need sleep.'
'Of course we do, yeah. Thanks, Donna,' he added as she handed him a steaming cup. She sat down diagonally across from him and leaned over her own tea, making a point of looking at him now. All her earlier befuddlement and discomfort turned into compassion as she took in his haggard appearance.
'Are you all right, though? 'Cause if you're not, you can tell me. I'll always listen.'
The Doctor was looking down into his tea, his throat bobbing before he replied 'Thank you, Donna, but I can't. It's not you, I just… I can't.'
He finally raised his eyes to meet hers then, and she could see shadows in their darkness, shadows that told of how old he really was. So much time and so much loss. Donna simply nodded and laid her hand on his arm, trying to communicate her sympathy and understanding without words. With a smile that trembled ever so slightly at its edges, the Doctor briefly put his hand over hers. She kept her hand in place as they sipped their tea in silence, and it was the most eloquent silence Donna had ever known.