Start with the obvious three
Before looking for anything unusual, check growth, sleep and food. A teenager growing quickly, sleeping seven hours and going ten hours between lunch and dinner on training day will be tired, and none of that is mysterious.
Teenage body clocks genuinely shift later during puberty, so falling asleep at ten is harder than it was, while school still starts at half eight. Eight to ten hours is the target and most teenagers are well short.
Where the day goes on a training day
Under-fuelling, the common culprit
The most frequent cause of persistent tiredness in a training teenager is simply not enough food for the amount she is doing, especially through a growth spurt when requirements jump.
The tell is tiredness that improves noticeably within a fortnight of adding a substantial pre-training snack and a proper post-training meal. That is worth trying before anything else.
Iron, once periods have started
Low iron is common in teenage girls, particularly with heavy periods, and particularly in athletes who eat little red meat. It causes exactly the picture parents describe: tired constantly, breathless in sessions, and performance that will not budge.
It needs a blood test rather than guesswork, and ask for ferritin to be included, because iron stores can be low before anaemia shows.
Cycle-related tiredness
A three-month record of cycle days against energy usually makes the pattern obvious, and makes it plannable.
- Fatigue in the days before a period is commonly reported.
- Heavy bleeding drains iron and energy directly.
- Broken sleep around a period adds to it.
- Pain and disturbed nights compound the following day's session.
Other causes worth a GP visit
- Thyroid problems.
- Coeliac disease or other absorption issues.
- Vitamin D deficiency.
- Glandular fever or a lingering viral illness.
- Low mood, anxiety, or exam-period stress.
- Overtraining across school and club with no easy weeks.
The order to work through
- Two weeks of proper fuelling around every session.
- Sleep protected at eight to ten hours, phones out of the bedroom.
- One genuinely easier week, and add up the real weekly training total.
- Still tired? GP appointment, and ask for full blood count, ferritin, thyroid and vitamin D.
- Mention periods, training hours and any injuries at that appointment.