Miley Cyrus has "horrific" hangovers.
The 20-year-old singer-and-actress suffers from tachycardia, a heart condition which friends say is being made worse by her constant partying.
A source told Heat magazine: "Miley's issue is basically benign, provided she leads a sober life. The problem is, she doesn't. She'll smoke most nights out and she makes no secret of her fondness of certain recreational pursuits. Miley thinks she's in control but she's not. Her attitude towards drugs reflects that.
"She has horrific mornings after, which are more like days. After the 'Bangerz' [album] launch party she stayed in bed with her dogs for a couple of days just getting her strength back."
Although 'We Can't Stop' singer Miley is inspired by Madonna, friends think she needs to realise how the 'Hung Up' singer manages to successfully balance her personal life with her raunchy career.
The source added: "Miley thinks she's found the formula for success: being outrageous and ridiculously sexual, so she's going to keep doing it.
"But she's not got the stamina to keep it up. Madonna is ridiculously fit and a genius at giving the impression she's having a wild time, when in fact she's in bed by 9pm."
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